Move default implementations into Device.

The current abstract class was a nice idea but has led to a lot of
copy & paste in practice. Right now, no one we know of has any extra
menu items, so let's make the default menu available to everyone.

(If we assume that someone somewhere really does need custom
device-specific menu options, a better API would let them add to
our menu rather than replacing it.)

Change-Id: I59f6a92f3ecd830c2ce78ce9da19eaaf472c5dfa