broadcaster: Update fallback device on active device change

While being in active and playing broadcast mode a update of active
device happens, fallback device needs to be updated. Fallback device
don't need to be updated if all broadcast are stopped.

Bug: 388625326
Bug: 375422795
Flag: leaudio_broadcast_primary_group_selection
Test: atest LeAudioBroadcastServiceTest
Change-Id: I2a17bbb2a04c35d9a64f037e385ff5b29b2a4983