PWM: disable HW overlays for PIE

SurfaceFlinger wont wake up the GPU in SysUI's fullscreen state,
probably because it does not occupy any tangible region. That sets poor
PIE into software rendering, which has bad effects on transparency and
mdp compositioning. Disabling HW overlays as long as PIE is active seems
like a good alternative for now.

Change-Id: Ie7ed720edc5bbdd0cfb79c34a22996cf15fcd79b