recovery_keys: update for CWM API changes

CWM's eliminating device_toggle_display() and
ui_get_showing_back_button() (see http://review.cyanogenmod.org/62724).
Remove these in our recovery_keys.c.

Signed-off-by: Alberto97 <albertop2197@gmail.com>

See also this: http://review.cyanogenmod.org/#/c/62900/