LeAudio: Do not attach not fully connected group member

AssignCIS is failing during the attach if the device is not considered
fully connected (there are still pending some initial operations like
charateristic reads). We should attach this device later, when fully
initialized and ready. Attach will be triggered when the device changes
state to CONNECTED.

Bug: 364993085
Test: atest bluetooth_le_audio_client_test
Flag: EXEMPT; regression coverred with unit tests; new test added

Change-Id: I90d6119ebb5dbf31c503d65aed9073c7b1a18742