skhdr::Agtm: Change fixed-length arrays to std::vectors

This was written with the idea that it wasn't going to be publicly
visible. Now that it will be publicly visible, it's more robust and
reasonable to just use a std::vector.

Bug: 468928417
Change-Id: I6168b31fbc285e0d76987e0063d0c3d4198f1f78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1137118
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>
4 files changed