Add factory for Android font manager.

This allows users to create an Android font manager with their own set
of fonts, or augment the system set. This will allow for removal of
the current globals which are used for a similar, but more constained,
purpose.

BUG=skia:2817,skia:3314,chromium:407340

Review URL: https://codereview.chromium.org/887113002
6 files changed