Add chromecast audio support.

This reportedly makes chromecast fully work for our nooks.
You still need to install MirrorEnabler referenced here:
http://forum.xda-developers.com/hardware-hacking/chromecast/experimental-enable-mirroring-device-t2812193

If you have no sound, reportedly erasing chromecast app preferences
fixed that.

Change-Id: I79b734afdbb5c39aebdc362571ec3e7f15949149