[RN] Add workaround for Android view clipping

Looks like Android gets confused as to what surface to blit when we hide or
show toolbars. Setting a border on the container, seems to force the entire
area to blit properly.

Other attempted approaches, with no success:
- zIndex of -100
- width and height of 0
- opacity of 0 and setting 'disabled' on touch containers

This patch applies the workaround in the welcome page and conference containers.

react/features/conference/components/styles.js

     conference: {
         alignSelf: 'stretch',
         backgroundColor: ColorPalette.appBackground,
+
+        // XXX These properties are a workaround for Android views clipping,
+        // RN doesn't properly blit our overlays on top of video views.
+        borderColor: ColorPalette.appBackground,
+        borderWidth: 0.2,
+
         flex: 1
     },
 

react/features/welcome/components/styles.js

     container: {
         alignSelf: 'stretch',
         backgroundColor: ColorPalette.blue,
+
+        // XXX These properties are a workaround for Android views clipping,
+        // RN doesn't properly blit our overlays on top of video views.
+        borderColor: ColorPalette.appBackground,
+        borderWidth: 0.2,
+
         flex: 1
     },