le_audio: Introduce connection state handling for LE Audio group

Patch implements connection state handling for LE Audio device group.

Tag: #feature
Test: Set LE audio device as active
Sponsor: jpawlowski@
Bug: 150670922
Merged-In: I11222ca07e265ac8b6dc3c21650874ebeffa473c
Change-Id: I11222ca07e265ac8b6dc3c21650874ebeffa473c