leaudio: Minor clean up

Make sure to use group_id from the callback and not the
active_group_id_.
For streaming indeed it shall be same number therefore adding assert

Bug: 150670922
Test: atest --host bluetooth_le_audio_client_test
sponsor: @jpawlowski
Change-Id: Idbfe4a135e00cba098ce49fdcde5c153f0a4deaa