Make various src/app code compile with -Wconversion

[bzbarsky-apple]

We don't actually enable -Wconversion for this code yet, because it's
compiled directly into example apps, so we would need to enable
-Wconversion for those, and they aren't quite there.

Problem

Various -Wconversion failures in src/app code.

Summary of Changes

Fix the failures.

[github-actions]

Size increase report for "nrfconnect-example-build" from 99aaed7

File	Section	File	VM
chip-lighting.elf	text	-4	-4
chip-lighting.elf	shell_root_cmds_sections	-12	-12

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lighting.elf and ./pull_artifact/chip-lighting.elf:

sections,vmsize,filesize
.debug_str,0,20
.debug_abbrev,0,16
.debug_line,0,4
.debug_frame,0,-4
text,-4,-4
.debug_info,0,-8
shell_root_cmds_sections,-12,-12
.debug_loc,0,-92

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.debug_str,0,20
.debug_abbrev,0,16
.debug_line,0,8
.debug_frame,0,-4
.debug_info,0,-8
.debug_loc,0,-96

[github-actions]

Size increase report for "esp32-example-build" from 99aaed7

File	Section	File	VM
chip-wifi-echo.elf	.flash.rodata	80	80
chip-wifi-echo.elf	.flash.text	4	4

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-wifi-echo.elf and ./pull_artifact/chip-wifi-echo.elf:

sections,vmsize,filesize
.debug_loc,0,240
.debug_line,0,102
.flash.rodata,80,80
.debug_info,0,71
.debug_str,0,20
.flash.text,4,4
.xt.prop._ZN4chip8Encoding12LittleEndian7Write32ERPhj,0,2
.debug_abbrev,0,-3
[Unmapped],0,-80