Stop memory management on blanking overlay graphics

This prevents a crash in TWRP when trying to blank
the screen on devices that are using overlay graphics.

Change-Id: I155868616ffa1b211e97be97e4abe8b03dad7ca0
Allow using variables and addition/subtraction in variables

Change-Id: I1acba95302e414e1a471d27474508a7c48df0328