Fix handling of postrolls

adsLoader.contentComplete was called twice which led to postrolls not seeing
the STARTED event.

Also make the onPositionDiscontinuity handler easier to follow by renaming
variables, and remove the adGroupIndex field.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161537765

demo/src/main/java/com/google/android/exoplayer2/demo/PlayerActivity.java

@@ -336,8 +336,6 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
         }
         }
         try {
         try {
           mediaSource = createAdsMediaSource(mediaSource, Uri.parse(adTagUriString));
           mediaSource = createAdsMediaSource(mediaSource, Uri.parse(adTagUriString));
-          // The demo app has a non-null overlay frame layout.
-          simpleExoPlayerView.getOverlayFrameLayout().addView(adOverlayViewGroup);
         } catch (Exception e) {
         } catch (Exception e) {
           showToast(R.string.ima_not_loaded);
           showToast(R.string.ima_not_loaded);
         }
         }