refactor ui functions into a class

Move all the functions in ui.c to be members of a ScreenRecoveryUI
class, which is a subclass of an abstract RecoveryUI class.  Recovery
then creates a global singleton instance of this class and then invoke
the methods to drive the UI.  We use this to allow substitution of a
different RecoveryUI implementation for devices with radically
different form factors (eg, that don't have a screen).

Change-Id: I7fd8b2949d0db5a3f47c52978bca183966c86f33