Only load system typefaces that do not exist in theme

Change-Id: I8dbe5a91a73d0df53a0ebc7b4e4461237288f0d9
Cache ThemedTypefaceHelpers for better performance

This fixes the jank seen when scrolling through the list of fonts
as well as the bad characters that show up from time to time.

Change-Id: I0002a92edb154276035015fc7836dd72ea43dbf7

Conflicts:
	src/org/cyanogenmod/theme/chooser/ChooserBrowseFragment.java