Make it possible to disable DownloadService notifications

Issue: #4389

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=204728270

demos/main/src/main/AndroidManifest.xml

@@ -79,7 +79,7 @@
     <service android:name="com.google.android.exoplayer2.demo.DemoDownloadService"
         android:exported="false">
       <intent-filter>
-        <action android:name="com.google.android.exoplayer.downloadService.action.INIT"/>
+        <action android:name="com.google.android.exoplayer.downloadService.action.RESTART"/>
         <category android:name="android.intent.category.DEFAULT"/>
       </intent-filter>
     </service>

library/core/src/main/java/com/google/android/exoplayer2/scheduler/RequirementsWatcher.java

@@ -87,7 +87,7 @@ public final class RequirementsWatcher {
   public void start() {
     Assertions.checkNotNull(Looper.myLooper());
 
-    checkRequirements(true);
+    requirementsWereMet = requirements.checkRequirements(context);
 
     IntentFilter filter = new IntentFilter();
     if (requirements.getRequiredNetworkType() != Requirements.NETWORK_TYPE_NONE) {
@@ -158,13 +158,11 @@ public final class RequirementsWatcher {
     }
   }
 
-  private void checkRequirements(boolean force) {
+  private void checkRequirements() {
     boolean requirementsAreMet = requirements.checkRequirements(context);
-    if (!force) {
-      if (requirementsAreMet == requirementsWereMet) {
-        logd("requirementsAreMet is still " + requirementsAreMet);
-        return;
-      }
+    if (requirementsAreMet == requirementsWereMet) {
+      logd("requirementsAreMet is still " + requirementsAreMet);
+      return;
     }
     requirementsWereMet = requirementsAreMet;
     if (requirementsAreMet) {
@@ -187,7 +185,7 @@ public final class RequirementsWatcher {
     public void onReceive(Context context, Intent intent) {
       if (!isInitialStickyBroadcast()) {
         logd(RequirementsWatcher.this + " received " + intent.getAction());
-        checkRequirements(false);
+        checkRequirements();
       }
     }
   }
@@ -198,14 +196,14 @@ public final class RequirementsWatcher {
     public void onAvailable(Network network) {
       super.onAvailable(network);
       logd(RequirementsWatcher.this + " NetworkCallback.onAvailable");
-      checkRequirements(false);
+      checkRequirements();
     }
 
     @Override
     public void onLost(Network network) {
       super.onLost(network);
       logd(RequirementsWatcher.this + " NetworkCallback.onLost");
-      checkRequirements(false);
+      checkRequirements();
     }
   }
 }

library/dash/src/test/java/com/google/android/exoplayer2/source/dash/offline/DownloadServiceDashTest.java

@@ -20,14 +20,12 @@ import static com.google.android.exoplayer2.source.dash.offline.DashDownloadTest
 import static com.google.android.exoplayer2.testutil.CacheAsserts.assertCacheEmpty;
 import static com.google.android.exoplayer2.testutil.CacheAsserts.assertCachedData;
 
-import android.app.Notification;
 import android.content.Context;
 import android.content.Intent;
 import android.net.Uri;
 import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.offline.DownloadAction;
 import com.google.android.exoplayer2.offline.DownloadManager;
-import com.google.android.exoplayer2.offline.DownloadManager.TaskState;
 import com.google.android.exoplayer2.offline.DownloadService;
 import com.google.android.exoplayer2.offline.DownloaderConstructorHelper;
 import com.google.android.exoplayer2.offline.StreamKey;
@@ -52,7 +50,6 @@ import org.junit.Before;
 import org.junit.Ignore;
 import org.junit.Test;
 import org.junit.runner.RunWith;
-import org.mockito.Mockito;
 import org.robolectric.RobolectricTestRunner;
 import org.robolectric.RuntimeEnvironment;
 import org.robolectric.annotation.Config;
@@ -135,18 +132,12 @@ public class DownloadServiceDashTest {
             dashDownloadManager.startDownloads();
 
             dashDownloadService =
-                new DownloadService(/*foregroundNotificationId=*/ 1) {
-
+                new DownloadService(DownloadService.FOREGROUND_NOTIFICATION_ID_NONE) {
                   @Override
                   protected DownloadManager getDownloadManager() {
                     return dashDownloadManager;
                   }
 
-                  @Override
-                  protected Notification getForegroundNotification(TaskState[] taskStates) {
-                    return Mockito.mock(Notification.class);
-                  }
-
                   @Nullable
                   @Override
                   protected Scheduler getScheduler() {