Bump third_party/openthread/repo from `e958f88` to `af5938e` #456

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 37m 13s
Build on Linux (fake, gcc_release, clang, simulated)	2h 13m 40s
Build on Linux (coverage)	17m 11s
Build on Linux (gcc_debug)	54m 47s
Build on Linux (python_lib)	31m 20s
	5h 34m 11s

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