Bump third_party/openthread/repo from `66eaeec` to `a8afffb` #1132

Job	Run time
Build on Linux (gcc_debug)	34m 36s
Build on Linux (fake, gcc_release, clang, simulated)	1h 15m 2s
Build on Linux (python_lib)	17m 36s
Build on Darwin (clang, python_lib, simulated)	31m 11s
Build on Linux (coverage)	9m 21s
Build on Linux (python lighting-app)	6m 14s
	2h 54m 0s

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