Bump third_party/openthread/repo from `aed9cd1` to `bb5e9b7` #1576

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 30m 45s
Build on Linux (fake, gcc_release, clang, simulated)	2h 1m 13s
Build on Linux (coverage)	13m 40s
Build on Linux (gcc_debug)	1h 1m 33s
Build on Linux (python_lib)	29m 19s
Build on Linux (python lighting-app)	10m 33s
	6h 27m 3s

Provide feedback