le_audio: Avoid activation unicast and broadcast at the same time

A pending state is introducted to proper maintain switching of LE Audio
devices e.g. Unicast <-> Broadcast. While one type device is activating,
second should be aware of such process. Without knowledge if
device/group is activating it may happen that service will call stack to
activate broadcast and unicast at the same time which is not supported
by now, especially by audio framework side.

Tag: #feature
Bug: 329169082
Bug: 316005152
Bug: 329178567
Test: atest bluetooth_le_audio_test
Test: atest LeAudioService
Change-Id: I8c7ea4d0dc3038e6e204e3fa1286fc2c6e3f74b9