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/overlay/SurveyFeatureProvider.java

@@ -20,6 +20,7 @@ import android.content.BroadcastReceiver;
 import android.content.Context;
 import android.content.IntentFilter;
 import android.support.annotation.Nullable;
+import android.support.v4.content.LocalBroadcastManager;
 
 /**
  * An interface for classes wishing to provide the ability to serve surveys to implement.
@@ -81,5 +82,11 @@ public interface SurveyFeatureProvider {
      * after a call to {@link #createAndRegisterReceiver(Activity)}.
      * @param activity The activity that was used to register the BroadcastReceiver.
      */
-    void unregisterReceiver(Activity activity, BroadcastReceiver receiver);
+    static void unregisterReceiver(Activity activity, BroadcastReceiver receiver) {
+        if (activity == null) {
+            throw new IllegalStateException("Cannot unregister receiver if activity is null");
+        }
+
+        LocalBroadcastManager.getInstance(activity).unregisterReceiver(receiver);
+    }
 }

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;
         }
     }
 }