Use UTF32 directly in HarfBuzz<->Skia interface.

This fixes a TODO to use kUTF32_TextEncoding directly. This avoids
encoding UTF32 codepoints from HarfBuzz into UTF16 to pass to Skia,
which will then re-decode to UTF32. This also removes a use of
SkUtils.h which Skia desires to move to private use.

Change-Id: I087f4216c25f7874e496920cd33e2f109530c397