[rust png] Restrict `libpng` visibility to `libwebp:cwebp` exe.

The `libwebp` **library** doesn't depend on `libpng`.  This CL makes
this more explicit, by tweaking visibilities of GN rules that cover
`libwebp` and `libpng` libraries.  In particular, this CL:

* Only allows depending on `libpng` from `libwebp:imagedec`.  This
  consequently disallows depending on `libpng` from other parts of
  `libwebp` (e.g. from the `libwebp` library).  This consequence is
  the main motivation/goal for this CL.
* Only allows depending on `libwebp:imagedec` from `libwebp:cwebp`
  (the latter is an `executable` target)
* Only allows depending on `libwebp:cwebp` from specific directories
  related to Android builds
* Deletes the `libwebp:dwebp` `executable` target (which was not used
  elsewhere in Chromium).  And consequently delete the
  `libwebp:imageenc` target which was only used from `libwebp:dwebp`.

Bug: 443128323
Change-Id: I28dc6553593c6db6e9317d3dd7e828ca093cf3ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6977689
Reviewed-by: James Zern <jzern@google.com>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523731}
NOKEYCHECK=True
GitOrigin-RevId: 269786b942d89b7695fbb89550fc191d1f2079d4