Themes: Make parse() method in FontListParser public

This allows other apps, such as the Theme Chooser, to use the parser
without needing to duplicate code.  Methods were generalized to use
input streams that support marking so that the stream can be reset
after checking if the xml is in the legacy format.

Change-Id: Id77adabba836267e3dfcdd536b2859d3faa7d78d
REF: CHOOSER-83
(cherry picked from commit 0d9fa435)