Inject mocks in RemoteVolumeControllerTest via constructor

This fix avoids the test wrongfully initializing real components before
the tests can modify mockable fields, which causes NPEs.

Bug: 318078730
Test: atest RemoteVolumeGroupControllerTest
Change-Id: I350e3e2e45eadb0f9737ba5d4b45c9f80e3355fb

src/com/android/settings/notification/RemoteVolumeGroupController.java

@@ -16,6 +16,8 @@
 
 package com.android.settings.notification;
 
+import android.annotation.NonNull;
+import android.annotation.Nullable;
 import android.content.Context;
 import android.content.Intent;
 import android.media.MediaRouter2Manager;
@@ -53,6 +55,7 @@ public class RemoteVolumeGroupController extends BasePreferenceController implem
     @VisibleForTesting
     static final String SWITCHER_PREFIX = "OUTPUT_SWITCHER";
 
+    @Nullable
     private PreferenceCategory mPreferenceCategory;
     private final List<RoutingSessionInfo> mRoutingSessionInfos = new ArrayList<>();
 
@@ -61,6 +64,7 @@ public class RemoteVolumeGroupController extends BasePreferenceController implem
     @VisibleForTesting
     MediaRouter2Manager mRouterManager;
 
+    // Called via reflection from BasePreferenceController#createInstance().
     public RemoteVolumeGroupController(Context context, String preferenceKey) {
         super(context, preferenceKey);
         if (mLocalMediaManager == null) {
@@ -71,6 +75,19 @@ public class RemoteVolumeGroupController extends BasePreferenceController implem
         mRouterManager = MediaRouter2Manager.getInstance(context);
     }
 
+    @VisibleForTesting
+    /* package */ RemoteVolumeGroupController(
+            @NonNull Context context,
+            @NonNull String preferenceKey,
+            @NonNull LocalMediaManager localMediaManager,
+            @NonNull MediaRouter2Manager mediaRouter2Manager) {
+        super(context, preferenceKey);
+        mLocalMediaManager = localMediaManager;
+        mRouterManager = mediaRouter2Manager;
+        mLocalMediaManager.registerCallback(this);
+        mLocalMediaManager.startScan();
+    }
+
     @Override
     public int getAvailabilityStatus() {
         if (mRoutingSessionInfos.isEmpty()) {