Adding an overlay for first time Cast users plus volume slider

This address two of the UX guideline bugs:

- Shows a touch volume slider. When the user touches the screen during Chromecast playback, a volume slider will appear for a few seconds, allowing the user to set the volume. 

- The first time a Chromecast is detected on the network, shows an overlay highlighting the cast button. The overlay can be dismissed by touching anywhere on the screen.