Support variable font family

A variable font family sets the requested weight and italic style
axes before passing down to the rendering pipeline.
Also, add more accessors to the PositionedGlyph APIs for knowing
fake bold/italic information as well as wght/ital overrides.

Bug: 281769620
Test: minikin_tests
Test: atest CtsTextTestCases:android.text.cts.VariableFamilyTest

Change-Id: I4a4770bf185a1c21113a293fe3d831573411ec26