ref(video-layout): remove instance variable for gating show/hide

The instance variable is not accurate. By default isVisible is
set to false but nothing sets the video container to actually
not be visible. As such it is possible for the video element
itself to autoplay, thereby making video visible, while the
isVisible boolean is still false. The fix chosen is to remove
instance variable and always respect calls to show/hide so
that the video container can be set to hidden.

modules/UI/videolayout/VideoContainer.js

         this.emitter = emitter;
         this.resizeContainer = resizeContainer;
 
-        this.isVisible = false;
-
         /**
          * Whether the background should fit the height of the container
          * (portrait) or fit the width of the container (landscape).
      * TODO: refactor this since Temasys is no longer supported.
      */
     show() {
-        // its already visible
-        if (this.isVisible) {
-            return Promise.resolve();
-        }
-
         return new Promise(resolve => {
             this.$wrapperParent.css('visibility', 'visible').fadeTo(
                 FADE_DURATION_MS,
                 1,
                 () => {
-                    this.isVisible = true;
                     resolve();
                 }
             );
         // hide its avatar
         this.showAvatar(false);
 
-        // its already hidden
-        if (!this.isVisible) {
-            return Promise.resolve();
-        }
-
         return new Promise(resolve => {
             this.$wrapperParent.fadeTo(FADE_DURATION_MS, 0, () => {
                 this.$wrapperParent.css('visibility', 'hidden');
-                this.isVisible = false;
                 resolve();
             });
         });