Accumulative fixes for LE audio test app

1. Cannot invoke setValue on a background thread from Coroutine
2. Broadcast sources refresh issue
3. fix NPE when getSelectedItem is null
4. Fix issue removing broadcast source with incorrect sourceId
5. Adding debug message

Test: presubmit
Change-Id: Ie8511b84b2b21e3ed17a204a843d18944399d6e4