Expose current scrubber position through onScrubStart

Issue: #2910

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158149904

library/ui/src/main/java/com/google/android/exoplayer2/ui/DefaultTimeBar.java

@@ -442,7 +442,7 @@ public class DefaultTimeBar extends View implements TimeBar {
       parent.requestDisallowInterceptTouchEvent(true);
     }
     if (listener != null) {
-      listener.onScrubStart(this);
+      listener.onScrubStart(this, getScrubberPosition());
     }
   }
 

library/ui/src/main/java/com/google/android/exoplayer2/ui/PlaybackControlView.java

@@ -1045,7 +1045,7 @@ public class PlaybackControlView extends FrameLayout {
       OnClickListener {
 
     @Override
-    public void onScrubStart(TimeBar timeBar) {
+    public void onScrubStart(TimeBar timeBar, long position) {
       removeCallbacks(hideAction);
       scrubbing = true;
     }

library/ui/src/main/java/com/google/android/exoplayer2/ui/TimeBar.java

@@ -95,8 +95,9 @@ public interface TimeBar {
      * Called when the user starts moving the scrubber.
      *
      * @param timeBar The time bar.
+     * @param position The position of the scrubber, in milliseconds.
      */
-    void onScrubStart(TimeBar timeBar);
+    void onScrubStart(TimeBar timeBar, long position);
 
     /**
      * Called when the user moves the scrubber.