ActivityManager: Rejigger HighEndGfx: only propget/stringcomp and resget once (in isHighEnd, rather than constructor)

Change-Id: I6ec977a9e8c7f46b186594c7bf6cad6a690121c9
Merge "ActivityManager: Rejigger HighEndGfx: only propget/stringcomp and resget once (in isHighEnd, rather than constructor)" into pac-4.4