Replace Skia font macros with enums.

This mechanically replaces kXXX_SkTextEncoding with SkTextEncoding::kXXX
and kXXX_SkFontHinting with SkFontHinting::kXXX. This will allow Skia to
remove these old macro constants and get everyone on the new enums.

Test: refactoring CL. Existing unit tests still pass.
Change-Id: I4c3b1834b1c4cdfc7f708613c49dfb18fc3194f5