Bump third_party/openthread/repo from `aed9cd1` to `3a28b7b` #328

Job	Run time
REPL Tests - Linux	1h 30m 16s
Test Suites - Darwin (no-ble-asan-clang)	1h 24m 58s
Test Suites - Linux (no-ble-tsan-clang)	1h 47m 7s
Test Suites - Darwin (no-ble-tsan-clang)	2h 5m 1s
REPL Tests - Darwin	0s
	6h 47m 22s

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