Bump third_party/openthread/repo from `2aeb8b8` to `a147485` #403

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 44s
Build on Linux (gcc_debug)	1h 0m 49s
Build on Darwin (clang, python_lib, simulated)	2h 30m 5s
Build on Linux (coverage)	13m 56s
Build on Linux (python_lib)	22m 21s
Build on Linux (python lighting-app)	5m 51s
	6h 11m 46s

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