bass: Monitor LE Audio unicast source when managed

Assistant should notify about its activity when sink device has its
localy managed external source in receive state. Activity callback is
used to keep monitoring of unicast source streaming activity. Monitoring
source activity let unicast have knowledge about being in handover mode.

Bug: 360051130
Bug: 378661060
Test: atest LeAudioServiceTest
Test: atest BassClientServiceTest
Flag: com.android.bluetooth.flags.leaudio_monitor_unicast_source_when_managed_by_broadcast_delegator
Change-Id: I555e8a8027c14df57e8278afdcf896ad8ee386b6