le_audio: Sync on main loop on MicrophoneAttachToCurrentMediaScenario

Schedule setting active group in main thread to not introduce races with
resume call which may potentially lead to use after free issue.

Tag: #feature
Bug: 264233314
Test: atest bluetooth_le_audio_test
Change-Id: Ib69a8fe9c5e63b66c7c6963b27ca302263234ff2