Use player.retry in demo app instead of manual position tracking.

We currently do manual position tracking while the player is in error mode.
This is to allow an initial seek to the new position when trying to re-prepare.

Instead, we can just use our player.retry method and remove the tracking code.

PiperOrigin-RevId: 243819580

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

@@ -311,7 +311,7 @@ public class PlayerActivity extends AppCompatActivity
 
   @Override
   public void preparePlayback() {
-    initializePlayer();
+    player.retry();
   }
 
   // PlaybackControlView.VisibilityListener implementation
@@ -642,21 +642,11 @@ public class PlayerActivity extends AppCompatActivity
     }
 
     @Override
-    public void onPositionDiscontinuity(@Player.DiscontinuityReason int reason) {
-      if (player.getPlaybackError() != null) {
-        // The user has performed a seek whilst in the error state. Update the resume position so
-        // that if the user then retries, playback resumes from the position to which they seeked.
-        updateStartPosition();
-      }
-    }
-
-    @Override
     public void onPlayerError(ExoPlaybackException e) {
       if (isBehindLiveWindow(e)) {
         clearStartPosition();
         initializePlayer();
       } else {
-        updateStartPosition();
         updateButtonVisibility();
         showControls();
       }