Bump third_party/openthread/repo from `2aeb8b8` to `ff19e03` #399

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 12m 22s
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 54s
Build on Linux (coverage)	14m 40s
Build on Linux (gcc_debug)	1h 2m 51s
Build on Linux (python_lib)	23m 14s
Build on Linux (python lighting-app)	6m 57s
	5h 58m 58s

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