Cleaning codepoints/grapheme mess; it also helps with ICU API

Renamed all codepoints into utf16Index

Change-Id: Ie915395a56ac825637f6dbb25824cd1635a5b0a6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296438
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Julia Lavrova <jlavrova@google.com>
9 files changed