Add a proper TV UI

[Will Lunniss]

Add a new way to browse content when running on a TV using https://developer.android.com/tools/support-library/features.html#v17-leanback when supports Jelly Bean MR1+

Won't be compatiable with legacy Google TV which will be stuck with the old UI.

Required for Android TV support and important for generic TV/STB devices and Amazon Fire TV.