Use GetAccessingFabricIndex in a few more places.

[bzbarsky-apple]

People keep copy/pasting the
GetExchangeContext()->GetSessionHandle()->AsSecureSession()->GetFabricIndex()
when they really don't need it....

Problem

Unsafe code being used. In particular, that code would would crash if one of the scenes commands were groupcast, because AsSecureSession() would crash. Need more tests, clearly.

Change overview

Use the existing GetAccessingFabricIndex() API on CommandHandler, which should do the right thing.

Testing

Not sure how to test the scenes commands with groupcast. @jepenven-silabs do you know offhand?

[github-actions]

PR #13395: Size comparison from 5d0c314 to a6ae3ea

Increases (1 build for nrfconnect)

platform	target	config	section	5d0c314	a6ae3ea	change	% change
nrfconnect	lighting-app	nrf5340dk_nrf5340_cpuapp	text	550256	550260	4	0.0

Decreases (6 builds for esp32, k32w, linux, mbed, p6)

platform	target	config	section	5d0c314	a6ae3ea	change	% change
esp32	all-clusters-app	c3devkit	(read only)	891904	891832	-72	-0.0
			.flash.text	891904	891832	-72	-0.0
		m5stack	(read only)	951843	951791	-52	-0.0
			.flash.text	946459	946407	-52	-0.0
k32w	light	k32w061+release	(read/write)	655324	655292	-32	-0.0
			.text	570900	570868	-32	-0.0
linux	thermostat-no-ble	arm64	(read only)	2033804	2033660	-144	-0.0
			.text	1691168	1691024	-144	-0.0
mbed	all-clusters-app	CY8CPROTO_062_4343W+release	(read/write)	2348560	2348496	-64	-0.0
			.text	1311136	1311072	-64	-0.0
p6	all-clusters-app	default	(read/write)	2401656	2401592	-64	-0.0
			.text	1359920	1359856	-64	-0.0

Full report (31 builds for efr32, esp32, k32w, linux, mbed, nrfconnect, p6, qpg, telink)

platform	target	config	section	5d0c314	a6ae3ea	change	% change
efr32	lighting-app	BRD4161A	(read only)	829244	829244	0	0.0
			(read/write)	126996	126996	0	0.0
			.bss	125120	125120	0	0.0
			.data	1876	1876	0	0.0
			.text	829236	829236	0	0.0
		BRD4161A+rpc	(read only)	816888	816888	0	0.0
			(read/write)	143656	143656	0	0.0
			.bss	141680	141680	0	0.0
			.data	1976	1976	0	0.0
			.text	816880	816880	0	0.0
	window-app	BRD4161A	(read only)	802692	802692	0	0.0
			(read/write)	125936	125936	0	0.0
			.bss	124104	124104	0	0.0
			.data	1832	1832	0	0.0
			.text	802684	802684	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	891904	891832	-72	-0.0
			(read/write)	1314010	1314010	0	0.0
			.dram0.bss	69464	69464	0	0.0
			.dram0.data	14236	14236	0	0.0
			.flash.rodata	177248	177248	0	0.0
			.flash.text	891904	891832	-72	-0.0
			.iram0.text	62254	62254	0	0.0
		m5stack	(read only)	951843	951791	-52	-0.0
			(read/write)	445592	445592	0	0.0
			.dram0.bss	73960	73960	0	0.0
			.dram0.data	34064	34064	0	0.0
			.flash.rodata	206560	206560	0	0.0
			.flash.text	946459	946407	-52	-0.0
			.iram0.text	122671	122671	0	0.0
k32w	light	k32w061+release	(read/write)	655324	655292	-32	-0.0
			.bss	76776	76776	0	0.0
			.data	1848	1848	0	0.0
			.text	570900	570868	-32	-0.0
	lock	k32w061+release	(read/write)	659616	659616	0	0.0
			.bss	77072	77072	0	0.0
			.data	1868	1868	0	0.0
			.text	574876	574876	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	7056652	7056652	0	0.0
			(read/write)	326113	326113	0	0.0
			.bss	54865	54865	0	0.0
			.data	1096	1096	0	0.0
			.data.rel.ro	209392	209392	0	0.0
			.dynamic	560	560	0	0.0
			.got	57072	57072	0	0.0
			.init	24	24	0	0.0
			.init_array	168	168	0	0.0
			.rodata	384564	384564	0	0.0
			.text	5978228	5978228	0	0.0
	thermostat-no-ble	arm64	(read only)	2033804	2033660	-144	-0.0
			(read/write)	145089	145089	0	0.0
			.bss	64657	64657	0	0.0
			.data	880	880	0	0.0
			.data.rel.ro	72624	72624	0	0.0
			.dynamic	560	560	0	0.0
			.got	4000	4000	0	0.0
			.init	24	24	0	0.0
			.init_array	296	296	0	0.0
			.rodata	128988	128988	0	0.0
			.text	1691168	1691024	-144	-0.0
mbed	all-clusters-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2348560	2348496	-64	-0.0
			.bss	188724	188724	0	0.0
			.data	5312	5312	0	0.0
			.text	1311136	1311072	-64	-0.0
	lighting-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2330752	2330752	0	0.0
			.bss	180544	180544	0	0.0
			.data	5552	5552	0	0.0
			.text	1293352	1293352	0	0.0
	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2303904	2303904	0	0.0
			.bss	179592	179592	0	0.0
			.data	5544	5544	0	0.0
			.text	1266504	1266504	0	0.0
	pigweed-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	1140008	1140008	0	0.0
			.bss	11756	11756	0	0.0
			.data	4368	4368	0	0.0
			.text	103392	103392	0	0.0
	shell	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2054256	2054256	0	0.0
			.bss	157060	157060	0	0.0
			.data	4864	4864	0	0.0
			.text	1016856	1016856	0	0.0
nrfconnect	lighting-app	nrf52840dk_nrf52840	(read/write)	937115	937115	0	0.0
			bss	118112	118112	0	0.0
			rodata	108120	108120	0	0.0
			text	633324	633324	0	0.0
		nrf52840dk_nrf52840+rpc	(read/write)	923511	923511	0	0.0
			bss	115156	115156	0	0.0
			rodata	101548	101548	0	0.0
			text	628636	628636	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	848094	848094	0	0.0
			bss	116004	116004	0	0.0
			rodata	101296	101296	0	0.0
			text	550256	550260	4	0.0
	lock-app	nrf52840dk_nrf52840	(read/write)	909259	909259	0	0.0
			bss	117300	117300	0	0.0
			rodata	103392	103392	0	0.0
			text	611188	611188	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	820418	820418	0	0.0
			bss	115220	115220	0	0.0
			rodata	96620	96620	0	0.0
			text	528164	528164	0	0.0
	pigweed-app	nrf52840dk_nrf52840	(read/write)	542351	542351	0	0.0
			bss	52588	52588	0	0.0
			rodata	50668	50668	0	0.0
			text	376892	376892	0	0.0
	pump-app	nrf52840dk_nrf52840	(read/write)	910523	910523	0	0.0
			bss	117060	117060	0	0.0
			rodata	103608	103608	0	0.0
			text	612404	612404	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	(read/write)	907339	907339	0	0.0
			bss	117088	117088	0	0.0
			rodata	102864	102864	0	0.0
			text	609936	609936	0	0.0
	shell	nrf52840dk_nrf52840	(read/write)	797935	797935	0	0.0
			bss	109768	109768	0	0.0
			rodata	78148	78148	0	0.0
			text	533508	533508	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	710750	710750	0	0.0
			bss	107656	107656	0	0.0
			rodata	72448	72448	0	0.0
			text	451188	451188	0	0.0
p6	all-clusters-app	default	(read/write)	2401656	2401592	-64	-0.0
			.bss	116804	116804	0	0.0
			.data	2592	2592	0	0.0
			.text	1359920	1359856	-64	-0.0
	light-app	default	(read/write)	2323680	2323680	0	0.0
			.bss	105672	105672	0	0.0
			.data	2384	2384	0	0.0
			.text	1281944	1281944	0	0.0
	lock-app	default	(read/write)	2295904	2295904	0	0.0
			.bss	104552	104552	0	0.0
			.data	2336	2336	0	0.0
			.text	1254168	1254168	0	0.0
qpg	lighting-app	qpg6105+debug	(read only)	533192	533192	0	0.0
			(read/write)	146936	146936	0	0.0
			.bss	86624	86624	0	0.0
			.data	1004	1004	0	0.0
			.text	527872	527872	0	0.0
	lock-app	qpg6105+debug	(read only)	504968	504968	0	0.0
			(read/write)	146940	146940	0	0.0
			.bss	85760	85760	0	0.0
			.data	952	952	0	0.0
			.text	499648	499648	0	0.0
	persistent-storage-app	qpg6105+debug	(read only)	106448	106448	0	0.0
			(read/write)	146938	146938	0	0.0
			.bss	36146	36146	0	0.0
			.data	288	288	0	0.0
			.text	101128	101128	0	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	834522	834522	0	0.0
			bss	86924	86924	0	0.0
			noinit	37160	37160	0	0.0
			text	582766	582766	0	0.0