Move UTF16 helpers from SkUtils to SkUTF

Also delete the unused/deprecated functions
 - SkUTF8_NextUnichar
 - SkUTF16_NextUnichar
 - SkUTFN_CountUnichars
 - SkUTFN_Next

Change-Id: Icbcc6961cbc3112af6794742af03adb14369c45d
Bug: skia:12584
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/467123
Reviewed-by: Ben Wagner <bungeman@google.com>
11 files changed