BluetoothLeAudio: Improve handling lead device disconnection

To improve scenario when lead Le Audio device is disconnected for the
streaming group, while there are still other devices streaming,
LeAudioService will not notify audio framework or other users about
Le Audio lead device disconnection. Instead we try to reconnect under the hood
and keep using lead device as a audio device indetifier in the audio framework
in order to not stop the stream.

Bug: 150670922
Bug: 228809876
Test: atest BluetoothInstrumentationTests
Sponsor: @jpawlowski
Change-Id: I02b960c6a5a966b6650dc3485c5d3f6735e4566b
Merged-In: I02b960c6a5a966b6650dc3485c5d3f6735e4566b
(cherry picked from commit eaf4f412)