leaudio: Add graceful disconnect support.

With this patch, if user request to disconnect profile, streaming
is closed first and later profile disconnects from GATT.

We need to do it as disconnection from GATT does not assure ACL will be
disconnected, as there might be other profiles enabled on that ACL.

Sponsor: jpawlowski@
Bug: 205791687
Test: atest bluetooth_le_audio_client_test
Change-Id: Idf5f22c9bc5fe35e2ab42679a3db93115f5987ad