Fix issue 2811538: System server crash when disconnecting BT headset after using SCO off call.

Problem:
When the bluetooth device is removed, the AudioService clears all active SCO connections
and unlinks from the client application's binder interface death.
The problem is that the unlinking is done even if no more connections are active for a given client,
which throws a runtime exception that is not catched causing the system server to crash.

The fix consists in calling unlinkToDeath() in ScoClient.clearCount() only if the number of
active SCO connections for this client is not 0. The NoSuchElementException exception is also
catched when calling unlinkToDeath()

Change-Id: I7086424301fc63a5666da61c38169349d3e078f4