memoize activity manager graphics acceleration check

also account for devices without hardware rendering support

Change-Id: Ib400d85a261d9c5d0fd5770d6cda5f61832cba6b
allow forcing HighEndGfx mode

people have already started fipping the lomRam prop to enable
transparent status bar, this will enable it with fewer
repercussions

Change-Id: Ied6de65276425fa1e7db0bf9da35f1932a355172

Conflicts:
	core/java/android/app/ActivityManager.java