Revert "Switching SkText to UTF16"

This reverts commit a3262aa1d25eb31f118ee395a0ecae83fd0e27b6.

Reason for revert: I believe this is blocking the Flutter roll

Original change's description:
> Switching SkText to UTF16
>
> Required some changes in SkUnicode to support it
> SkShaper still works on UTF8
>
> Change-Id: I76645668e1d9bf95eb4539a066deea2b24ecf5e9
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/406360
> Reviewed-by: Julia Lavrova <jlavrova@google.com>
> Commit-Queue: Julia Lavrova <jlavrova@google.com>

TBR=bungeman@google.com,reed@google.com,jlavrova@google.com

Change-Id: I68b66dfbc4f3668a8fb765d5f557699e396cd3f5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/407177
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed