LeAudioService: Stick to chosen Lead device as long it is connected

If the device has been chosen as a lead, do not change it as long it is
connected even group become inactive.
This will improve active device change behavior

Bug: 291071502
Test: atest BluetoothInstrumentationTests
Test: manual active device switch while streaming
Tag: #feature
Change-Id: I3a9d951ffe0a5d7c6522bdc2963e86f9a4d519e3