bass_client: Enable source monitoring only for external broadcast

There is no need to enable source stream direction for unicast.

Tag: #Bug
Bug: 329172203
Bug: 339612732
Flag: com.android.bluetooth.flags.leaudio_use_audio_mode_listener
Test: atest BassClientServiceTest#testHandleUnicastSourceStreamStatusChange
Change-Id: Ib4ef31e120b40b01bdade6e05b496b3fa546e412