leaudio: Fix update pending contexts

We should use group_id and not active_group_id_ as it might differs e.g.
in case of active group change.
Also fix initialization of the pending_update_available_contexts_

Bug: 150670922
Test: atest --host bluetooth_le_audio_client_test
sponsor: @jpawlowski
Change-Id: If8e8040ae85dad4807883bf39a7b6bb167d3ff60