Refine AT command of BluetoothSinkAudioPolicy

- Change feature type from integer to string
- Only process expected AT+ANDROID commands in HeadsetStateMachine
- Do not intercept AT+ANDROID commands with unexpected format
- Handle unexpected commnads as normal vendor commands
- Refactor feature command to
    +ANDROID: (<feature1>),(<feature2>,arg1),(<feature3>,args...)
- Add more unit tests

Bug: 275668166
Bug: 280117645
Test: atest BluetoothInstrumentationTests
Change-Id: Ib502d1c51b26db41102e83a6720de2eaabdc9c63