Bump third_party/ot-br-posix/repo from `bd15f71` to `af26b01` #953

Job	Run time
Build framework (macos-release, -sdk macosx -configuration Release)	18m 14s
Build framework (ios-release, -sdk iphoneos -configuration Release)	10m 23s
Build framework (tvos-debug, -sdk appletvos -configuration Debug)	9m 11s
Build framework (watchos-debug, -sdk watchos -configuration Debug)	15m 49s
Run framework tests (asan, -enableAddressSanitizer YES -enableUndefinedBehaviorSanitizer YES)	31m 3s
Run framework tests (tsan, -enableThreadSanitizer YES)	24m 47s
Run framework tests (leaks, ENABLE_LEAK_DETECTION=1)	33m 46s
	2h 23m 13s

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