allow forcing HighEndGfx mode

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

Add persist.sys.force_highendgfx=true for enable it with enabled lowram.

Change-Id: Ied6de65276425fa1e7db0bf9da35f1932a355172

Conflicts:

	core/java/android/app/ActivityManager.java