Fix context leak in AudioSwitchPreferenceController.

In this controller the context is stored into a global singleton but was
not cleared, thus leaking context and associated views.

Change-Id: I4247f8ff753bc0a331c6c81a0e4b5b4bc41588de
Bug: 80507279
Test: robotests
Test: inspected hprof before/after change

tests/robotests/src/com/android/settings/sound/AudioOutputSwitchPreferenceControllerTest.java

@@ -230,6 +230,7 @@ public class AudioOutputSwitchPreferenceControllerTest {
         verify(mLocalBluetoothManager.getEventManager()).registerCallback(
                 any(BluetoothCallback.class));
         verify(mContext).registerReceiver(any(BroadcastReceiver.class), any(IntentFilter.class));
+        verify(mLocalBluetoothManager).setForegroundActivity(mContext);
     }
 
     @Test
@@ -240,6 +241,7 @@ public class AudioOutputSwitchPreferenceControllerTest {
         verify(mLocalBluetoothManager.getEventManager()).unregisterCallback(
                 any(BluetoothCallback.class));
         verify(mContext).unregisterReceiver(any(BroadcastReceiver.class));
+        verify(mLocalBluetoothManager).setForegroundActivity(null);
     }
 
     @Test