Use own enum instead of SkTypeface::Style.

The current use of SkTypeface::Style and the naming of
Typeface::fSkiaStyle are misleading. Neither is used to actually
interact with Skia (which no longer uses the type in its API). Instead,
this type and field are used to track the values which will be exposed
through the jni layer which match the Android SDK values for the
constants on the android.graphics.Typeface type.

This change allows Skia to hide and eventually remove SkTypeface::Style,
as this is the last known external user. Additionally this allows
Android an easier path when updating the Typeface API in the future.

Test: refactoring CL. Existing unit tests still pass.
Change-Id: Ic820105a5b0d98727586fdc4a9a1c45e8ad40ff3