Update the system update animation.

Switch to a Wear-like intro/loop system. We don't have an intro yet,
but hopefully this will let Wear delete more code when they move to N.
Unlike them, we don't hard-code the number of frames: we just look to
see what we have available. We do hard-code the fps though.

Also add a graphics test mode so you can see a demo of the UI components
without having to actually apply an OTA.

Also fix a bug where default locale is null rather than en-US: it's
more useful to show _some_ text if we don't have a locale (which should
only be during development anyway).

Bug: http://b/26548285
Change-Id: I63422e3fef3c41109f924d96fb5ded0b3ae7815d