Leanback themes override AlertDialog styles from other themes.
Makes system alert dialogs, and alert dialogs from games running on ATV,
have a style more appropriate to television.
Final visuals pending.
Side button layout fixes DatePickerDialog and TimePickerDialog with
dpad navigation.
TODO: consider using side button layout for other AlertDialog cases
(long messages and lists).
Change-Id: I74e4e04d16d6854377580be95a87684a0385b04b
4c0cb8a526