Bump third_party/ot-br-posix/repo from `bd15f71` to `af26b01` #397

Job	Run time
Unit / Integration Tests (main)	17m 5s
Unit / Integration Tests (clang)	17m 7s
Unit / Integration Tests (mbedtls)	16m 18s
Unit / Integration Tests (rotating_device_id)	17m 19s
Unit / Integration Tests (icd)	45m 9s
	1h 52m 58s

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