Extensible Android font configuration parsing.

Extensible Android font configuration parsing.

The xml parsing of the Android font configuration is quite lax in what
it allows. This has lead to issues with forward compatibility and
extending the existing format. This has lead to some confusion about
what the actual format is and how a given proposed change will affect
existing files and readers.

The main issue this fixes is containment. Tags are now only recognized
at the correct levels in the correct containing tags. Tags which are
not recognized are properly skipped. Tags which accumulate character
data now only accumulate the character data in their own element as
opposed to all child elements.

Review URL: https://codereview.chromium.org/1138073002