Restrict visibility of libpng to current users

No new dependencies should be added on libpng: it is actively being
migrated to a rust version, and image processing calls should go through
skia instead of calling libpng directly. This CL enforces that by
restricting the visibility of libpng targets to the current set of
users, meaning any new users will require owners approval.

The separate visibility file serves as an easily-auditable record of
who is currently depending on libpng, and how that changes over time.

Bug: 433723342
Change-Id: I9e0c94fa1547c1478b849325aa6914e92bfaff9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6777532
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491588}
NOKEYCHECK=True
GitOrigin-RevId: 30284c444a33749e2a4396f9f047b090b7089a34