Add mutex and callback check on a2dp sink interface

pts-bot is flaky on some test and this crash was spotted:
```
JNI DETECTED ERROR IN APPLICATION: obj == null
backtrace:
  _JNIEnv::CallVoidMethod()
  android::bta2dp_connection_state_callback()
```

Test: atest pts-bot
Bug: 245578454
Change-Id: I2c2d624468be53aa11f2e33ffccdb77a5b98468a