clear recovery framebuffers on allocation; display icon right after ui_init

Make ui_init() clear the framebuffer memory it maps in so the user
isn't treated to a visible flash of random bits on recovery startup.
Call ui_set_background() (to show the installing icon) right after
ui_init() to display something while device_recovery_start() is
working (which can take a second or two on some devices).

Bug: 3145331
Change-Id: I11e7859fab5847370ea4f4932c3fb1558af26c5d