skhdr::Agtm: Add public metadata

Move skhdr::AdaptiveGlobalToneMap to a header in include/private, so it
can be used for IPC serialization in Chrome.

Add warnings at skhdr::Agtm that it will be removed.

Bug: 468928417
Change-Id: I54766c79a4fcbcb7c2999e75708b92d25a44b004
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1138816
Reviewed-by: Christopher Cameron <ccameron@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>