Small clean up to get ready for receive state based triggering.

1. Maintain a map from device to its source state as they could be different.
2. Ensure thread safety.

Test: atest
Bug: 398700619
Flag: com.android.settingslib.flags.audio_stream_media_service_by_receive_state
Change-Id: I056c80401c12401c78037eb15bed007eb592c12a