state_machine: Fix race on switching buds

With this patch we handle following case.
0. Group contains Device A and Device B
1. Group is streaming only to Device A (B is OFF)
2. Device B reconnects to the phone and attaches to the stream
   (Configuration has been started)
3. In the mean time, Device A disconnects.

With this patch, Device B will continue with configuration dispite
the Device A which just disconnects.

Bug: 333970914
Test: atest bluetooth_le_audio_test
Flag: EXEMPT, regression verified with unit tests, new test added
Change-Id: If0cf374601b787431782b93fd9c5600087067aab