LeAudioService: Fix handling of lead device disconnection

With this patch we make sure, 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
Tag: #feature

Change-Id: I7187801fe977610c759450dc6fa75d89c70db54a
Merged-In: I7187801fe977610c759450dc6fa75d89c70db54a