Fix broken notification (missing line in onStartCommand)

Also only listen to requirements when actually necessary, else we can
put the service into the foreground when it needn't be there.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=195597750

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

@@ -202,6 +202,7 @@ public abstract class DownloadService extends Service {
 
   @Override
   public int onStartCommand(Intent intent, int flags, int startId) {
+    lastStartId = startId;
     String intentAction = null;
     if (intent != null) {
       intentAction = intent.getAction();