Fix(UI): Update Play icon after seeking from end (shaka-project#3515)

We change the "play" button icon to become "rewind" when the video ends.
However, previously we did not update the icon again when seeking.
This would lead to the icon still looking like a "rewind" button if
the user seeks back from the end of the video.
This fixes that, by making the button listen to "seeking" events.

ui/play_button.js

@@ -52,6 +52,11 @@ shaka.ui.PlayButton = class extends shaka.ui.Element {
       this.updateIcon();
     });
 
+    this.eventManager.listen(this.video, 'seeking', () => {
+      this.updateAriaLabel();
+      this.updateIcon();
+    });
+
     this.eventManager.listen(this.adManager, AdManager.AD_PAUSED, () => {
       this.updateAriaLabel();
       this.updateIcon();