Fix starting the download service in the background throw exception

This happens when the device screen is locked.

This fixes a previous attempt to fix the problem.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=199130325

demos/main/src/main/java/com/google/android/exoplayer2/demo/SampleChooserActivity.java

@@ -94,16 +94,14 @@ public class SampleChooserActivity extends Activity
     SampleListLoader loaderTask = new SampleListLoader();
     loaderTask.execute(uris);
 
-    // Ping the download service in case it's not running (but should be).
-    Intent serviceIntent =
-        new Intent(this, DemoDownloadService.class).setAction(DownloadService.ACTION_INIT);
+    // Start the download service if it should be running but it's not currently.
     // Starting the service in the foreground causes notification flicker if there is no scheduled
     // action. Starting it in the background throws an exception if the app is in the background too
     // (e.g. if device screen is locked).
     try {
-      startService(serviceIntent);
+      DownloadService.start(this, DemoDownloadService.class);
     } catch (IllegalStateException e) {
-      startForegroundService(serviceIntent);
+      DownloadService.startForeground(this, DemoDownloadService.class);
     }
   }
 

library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadService.java

@@ -160,9 +160,9 @@ public abstract class DownloadService extends Service {
    * Starts the service, adding an action to be executed.
    *
    * @param context A {@link Context}.
-   * @param clazz The concrete download service being targeted by the intent.
+   * @param clazz The concrete download service to be started.
    * @param downloadAction The action to be executed.
-   * @param foreground Whether this intent will be used to start the service in the foreground.
+   * @param foreground Whether the service is started in the foreground.
    */
   public static void startWithAction(
       Context context,
@@ -177,6 +177,33 @@ public abstract class DownloadService extends Service {
     }
   }
 
+  /**
+   * Starts the service without adding a new action. If there are any not finished actions and the
+   * requirements are met, the service resumes executing actions. Otherwise it stops immediately.
+   *
+   * @param context A {@link Context}.
+   * @param clazz The concrete download service to be started.
+   * @see #startForeground(Context, Class)
+   */
+  public static void start(Context context, Class<? extends DownloadService> clazz) {
+    context.startService(new Intent(context, clazz).setAction(ACTION_INIT));
+  }
+
+  /**
+   * Starts the service in the foreground without adding a new action. If there are any not finished
+   * actions and the requirements are met, the service resumes executing actions. Otherwise it stops
+   * immediately.
+   *
+   * @param context A {@link Context}.
+   * @param clazz The concrete download service to be started.
+   * @see #start(Context, Class)
+   */
+  public static void startForeground(Context context, Class<? extends DownloadService> clazz) {
+    Intent intent =
+        new Intent(context, clazz).setAction(ACTION_INIT).putExtra(KEY_FOREGROUND, true);
+    Util.startForegroundService(context, intent);
+  }
+
   @Override
   public void onCreate() {
     logd("onCreate");