leaudio: Improve tracking cig state

With this patch, cig state is changed to enum so we can track if group
is idle, creating, created or removing.
Group shall not be removed while CIG is not in idle.
This patch also adds asserts to catch invalid behaviour

Bug: 226709330
Bug: 150670922
test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test
sponsor: @jpawlowski
Change-Id: If3a14dbf3c8283748c2ac1c43a13c94d4568fd8f