Simulator: improve simulation of transflective screens

This patch simulates the three possible states of a transflective
LCD: backlight on (bright screen as usual), backlight off (dimmed
screen) and LCD off (black screen).
Makes use of already defined 'BACKLIGHT_OFF_ALPHA'.

Change-Id: I9b0fc79b8d50c29e024ba1e6d9c2501119a7e0e0
2 files changed