Hide ICU C++ API from Skia users.

Parts of third_party need the C++ API so hide it from Skia users as
needed to prevent re-introduction.

This also avoids the ICU version renaming / name mangling when building
our own test version of ICU. This makes life in an editor and debugger
much easier.

Change-Id: I8fb1903e2b31e9dd04efa22173a03115d629c232
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/292854
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
5 files changed