Reorganize font enumeration API.

This CL cleans up APIs around font variation settings.
- Remove FontConfig and FontManager public API.
- Remove FontManagerService from system service.
- Extract inner class FontConfig.Axis as top-level class FontVariationAxis.
  This is used by Typeface.Builder public API to create new Typeface.
- Introduce and expose FontVariationAxis utility functions from/to string.
- Throws if the invalid font variation settings is passed.

Test: android.text.cts.FontVariationAxisTest passes
Test: android.graphics.cts.TypefaceTest passes
Test: android.graphics.cts.PaintTest passes
Change-Id: I9ccafe7a53935960566243e2856e166878ca59ae