New weight-aware font config

Parse new fonts.xml config file, and resolve weight selection based on
the base weight of the font (as defined by a weight alias specified in
the config file) and the requested bold flag. This change improves the
appearance of bold spans for alternate weights of Roboto.

In addition, this patch enables weight selection for fallback fonts.
For example, if an additional font with a weight of 100 is added to the
Hebrew font family in the fallback list, then requesting
"sans-serif-thin" would select that font for Hebrew text.

Bug: 14538154
Change-Id: I99a04fad4f7bf01c75726e760d42735dd9003496