LeAudio: Improve detecting disconnected devices.

When the device disconnection cleanup is ongoing, make
sure the group does not count devices which did not reached the
DISCONNECTED state yet. Those devices can be detected by their
connection ID being invalid.

Bug: 275154325
Test: atest --host bluetooth_le_audio_client_test bluetooth_le_audio_test --no-bazel-mode
Change-Id: I8417394a75646ea13c416587782eb6163acef239