Add support for caching font files in the Android SkFontMgr.

Add support for caching font files in the Android SkFontMgr.

SkFontMgr_Android lazily access font files from disk, which is not
possible when it is used in the renderer sandbox on Linux. Add a flag to
SkFontMgr_Android_CustomFonts for caching readonly FILE streams when
creating the font mgr.

Since the font mgr is created before the sandbox is initialized, it can
access these files on initialization, and use these cached streams safely
for its lifetime.

Review URL: https://codereview.chromium.org/1673373003