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

src/com/android/settings/sound/AudioSwitchPreferenceController.java

@@ -118,7 +118,6 @@ public abstract class AudioSwitchPreferenceController extends BasePreferenceCont
             Log.e(TAG, "Bluetooth is not supported on this device");
             return;
         }
-        mLocalBluetoothManager.setForegroundActivity(mContext);
         mProfileManager = mLocalBluetoothManager.getProfileManager();
     }
 
@@ -171,11 +170,13 @@ public abstract class AudioSwitchPreferenceController extends BasePreferenceCont
 
     @Override
     public void onStart() {
+        mLocalBluetoothManager.setForegroundActivity(mContext);
         register();
     }
 
     @Override
     public void onStop() {
+        mLocalBluetoothManager.setForegroundActivity(null);
         unregister();
     }