Cache ICU break iterators in SkUnicode_icu

Gate this behind an ifdef that can be disabled in environments where
the ICU ubrk_safeClone API is not available.

Change-Id: Ia1a3f677271622f2b7ae478b4d1ce76c74eed057
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352876
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: Jason Simmons <jsimmons@google.com>
3 files changed