LeAudioService: Fix handling of lead device disconnection

With this patch we make sure that Active device is cleared properly when
both all set members has been disconnected.
Especially in the use case, when as a first device the Lead device was
disconnected.

Bug: 233362628
Test: atest BluetoothInstrumentationTests

Change-Id: I7187801fe977610c759450dc6fa75d89c70db54a
(cherry picked from commit b20e13fc947f9d5434820e3a10ead62ce1d9973b)