Consolidate AssetManager calls in Font/FontFamily

The Font/FontFamily classes use private API calls in native
code to get the contents of an Asset.  These calls need to
be made on public APIs in order to isolate the graphics module
from the rest of the system.  Public Java APIs already exist so
this CL refactors the code to use those methods.

Test: CtsTextTestCases
Bug: 147136234
Change-Id: I35a6558b597684e59279d7a841669c5f909d5152