New xlarge screen size.

Not complete, only for experimentation at this point.

This includes a reworking of how screen size configurations are matched,
so that if you are on a larger screen we can select configurations for
smaller screens if there aren't any exactly matching the current screen.

The screen size at which we switch to xlarge has been arbitrarily
chosen; the compatibility behavior has not yet been defined.

Change-Id: I1a33b3818eeb51a68fb72397568c39ab040a07f5