Introduce FontStyle for future extensibility

This is a ground work for b/114479228
FontStyle is an argument of the font itemization. Currently Android
system uses weight and italic info for itemizing but maybe we will
use width and oblique style in future.
To be extensible in future, introduce FontStyle.

This CL includes:
- new class FontStyle which holds width and italic at this moment. In
  future, we may add width and oblique. (or grdual italic slant)
- Font class still returns weight/slant since they need to be immutable.
- Moved font weight constants from Font to FontStyle

Bug: 114479228
Test: atest CtsGraphicsTestCases
Change-Id: I0ba717fbf9f90ee6f760c1a0f6eda17b4bef6f43