Bump third_party/openthread/repo from `2aeb8b8` to `a147485` #404

Job	Run time
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 34m 22s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	2h 2m 9s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	2h 15m 37s
REPL Tests - Linux	1h 34m 22s
REPL Tests - Darwin	0s
	7h 26m 30s

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