sdk: introduce custom action keys

ATV targets often have special keys for apps, for example Netflix.

Set a reasonable set of defaults for atv. Remote keylayouts can
be configured to use these keycodes. If for some reason a device
has a keylayout that cannot be modified, the list can be overlaid.

Co-authored-by: Aaron Kling <webgeek1234@gmail.com>

Change-Id: Ied3ed5d68671148a3729b1f0e1a2b15c4b080239