Avoid onDestroy content in test

Even though `Activity.finish` is called by the @Rule before the teardown
happens, `onDestroy` may not be called synchronously and therefore may
happen after mocks have been nulled.

To prevent NPE, do not call `BroadcastDispatcher.unregisterReceiver` in
the test.

Test: atest com.android.systemui.controls
Fixes: 280020640
Change-Id: I992befeac6588116da03ed7869730f5b14b4e091