fix(large-video): center dominant speaker avatar using css

The vertical alignment was being set with javascript.
Recent changes might make the setting of alignment exit
early due to height 0 video. As position can be set
declaratively with css, use css to set position.

css/_videolayout_default.scss

     height: 300px;
     margin: auto;
     position: relative;
+    top: 50%;
+    transform: translateY(-50%);
 }
 
 #mixedstream {

modules/UI/videolayout/VideoContainer.js

         const { horizontalIndent, verticalIndent }
             = this.getVideoPosition(width, height, containerWidth, containerHeight);
 
-        // update avatar position
-        const top = (containerHeight / 2) - (this.avatarHeight / 4 * 3);
-
-        this.$avatar.css('top', top);
-
         this.$wrapper.animate({
             width,
             height,