ActivityManager: memoize for performance

isHighEndGfx and isLargeRam are frequently called upon.. isLargeRam very
frequently within the ActivityStack. these aren't going to change after
they're calculated the first time so subject them to a basic memoization.
FTW.

Change-Id: I176089f99ac6c6e4074cb2fde502c729986ea701
Use scaled font size for no recent apps text

Change-Id: Ib200928db09bcdd6661de5e450e2fc9c1b5566cf