Fix null pointer exception when surveys are turned off

Under certain conditions the provider could be null on
download completion which could lead to a
NPE if surveys are disabled. The method to remove
the receiver has been made static so an instance
is no longer required.

Test: robotests
Bug: 33707203
Change-Id: Icfb545697f24172db734dd7dad421796edf68186

src/com/android/settings/survey/SurveyMixin.java

@@ -74,9 +74,8 @@ public class SurveyMixin implements LifecycleObserver, OnResume, OnPause {
     public void onPause() {
         Activity activity = mFragment.getActivity();
         if (mReceiver != null && activity != null) {
-            SurveyFeatureProvider provider =
-                    FeatureFactory.getFactory(activity).getSurveyFeatureProvider(activity);
-            provider.unregisterReceiver(activity, mReceiver);
+            SurveyFeatureProvider.unregisterReceiver(activity, mReceiver);
+            mReceiver = null;
         }
     }
 }