Add qcom overlay graphics support to minui

graphics_overlay.cpp is based on a mix of AOSP code for
graphics_fbdev.cpp along with overlay graphics code from CAF that
was brought into earlier versions of TWRP and then further adapted
and improved before being brought into this patch set.

Also added a rule to build a minuitest binary for testing the
minui engine to ensure proper operation on a device.

Change-Id: I3972d3a6baa7002615319421ac07d9299c3cec69