[Audiosharing] Fix hysteresis mode

1. Move start broadcast logic from onPlaybackStarted to
   onBroadcastMetadataChanged
2. Move add source logic from onReceiveStateChanged(BIS>0) to
   onSourceAdded

Test: atest
Bug: 355222285
Bug: 377658982
Flag: com.android.settingslib.flags.audio_sharing_hysteresis_mode_fix
Change-Id: Ie0a2345e16caa2804fede930d174a67f2ec60259