[le audio] Broadcast only control primary group volume with setVolume

Previously we made a workaround to control all broadcast sink devices
with setVolume API.
Now systemUI is introducing the new support for separate group volume
control, we can update this to only control the primary group volume now.

Bug: 353451397
Bug: 354201794
Test: atest LeAudioServiceTest
Test: manual test with broadcast to confirm only primary volume changed
Change-Id: I70f0595cb2b793deb50b5df1077913177cb3eec5