[Audiosharing] Avoid fallback device update when onSourceRemoved.

Also check if there is playing broadcast before updating the fallback.

Updating fallback device during call audio will hit race condition in
broadcast-unicast switching. Considering there is no use case for
removing source in current product, we will disable fallback device
update when onSourceRemoved.

Bug: 329003470
Test: manual
Change-Id: I3038d2ecff91c26cf1b2fbda44476c8a8e31f4dd