Move all glyphsToUnichars to a single impl (rely on typeface)

- make it private in SkFont ('cause we hate this idea)
- remove from SkPaint as soon as we update callers

Bug: skia:8600
Change-Id: I8382b34b50b549d76ae4e849287994c4a8e1be87
Reviewed-on: https://skia-review.googlesource.com/c/175427
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>
10 files changed