Make SeekBar work properly with key events if focusable

This isn't perfect because it performs seeks whilst the
user is holding down L/R, rather than when they release
it. Performing the seek on release properly looks non
trivial, and would likely require extending SeekBar.

Issue: #2278

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143440580

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

@@ -735,8 +735,14 @@ public void onStartTrackingTouch(SeekBar seekBar) {
 
     @Override
     public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
-      if (fromUser && positionView != null) {
-        positionView.setText(stringForTime(positionValue(progress)));
+      if (fromUser) {
+        long position = positionValue(progress);
+        if (positionView != null) {
+          positionView.setText(stringForTime(position));
+        }
+        if (player != null && !dragging) {
+          seekTo(position);
+        }
       }
     }