Close unregistered clients with no PendingIntent

This may happen if a PendingIntent becomes invalid without
the client explicitly unregistering it (e.g. invalidated through
the Android system or canceled by the PendingIntent originator). This
avoids dangling client registrations at the service.

Bug: 117612105
Test: Compile only
Change-Id: Ic8d269b4c8d5267fc85170c0a95abcd03538fa9a