recovery_ui: 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_ui.c.

Change-Id: Ib26ffdf04194dc0f916934bf1bc1b92b47938ee2
Merge remote-tracking branch 'cyanogenmod/cm-11.0' into kk