Start downloads on DownloadService creation if there are no requirements

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=193662396

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

@@ -145,6 +145,8 @@ public abstract class DownloadService extends Service {
         requirementsWatcher =
             new RequirementsWatcher(getApplicationContext(), listener, requirements);
         requirementsWatcher.start();
+      } else {
+        downloadManager.startDownloads();
       }
     }
   }