Do not show PIP button if not allowed

There was a bug here that allowed the PIP button to be shown on any
track change event.  This bug only manifests on platforms without PIP,
such as Safari 12.

Found while working on #997 and #382

Change-Id: Iad51b2e9a5e86767a3ebf606c3e3b976d2d19ad7

ui/pip_button.js

@@ -211,7 +211,9 @@ shaka.ui.PipButton = class extends shaka.ui.Element {
    * @private
    */
   async onTracksChanged_() {
-    if (this.player && this.player.isAudioOnly()) {
+    if (!this.isPipAllowed_()) {
+      shaka.ui.Utils.setDisplay(this.pipButton_, false);
+    } else if (this.player && this.player.isAudioOnly()) {
       shaka.ui.Utils.setDisplay(this.pipButton_, false);
       if (document.pictureInPictureElement) {
         await document.exitPictureInPicture();