Feat(UI): Exit PiP when destroying UI

The UI can open up Picture-in-Picture mode, through the PiP button,
but it does not automatically leave PiP mode when being destroyed.
This CL adds leaving PiP mode to the destroy process.

Closes shaka-project#3553

Change-Id: I6830a7ceed7e4ca4fbd0e7d36b8257d564e0226c

ui/controls.js

@@ -198,6 +198,10 @@ shaka.ui.Controls = class extends shaka.util.FakeEventTarget {
    * @export
    */
   async destroy() {
+    if (document.pictureInPictureElement == this.localVideo_) {
+      await document.exitPictureInPicture();
+    }
+
     if (this.eventManager_) {
       this.eventManager_.release();
       this.eventManager_ = null;