Update to 1.6.34

Test: Covered by existing tests.

This is the same version we will use in Chromium:
https://chromium-review.googlesource.com/c/chromium/src/+/704209

https://github.com/glennrp/libpng/commit/edef058e180be435afd8feebb742a22360a7fa3f
(included in this revision) moved the SSE opts directly into the main
libpng source, so contrib/intel/intel_sse.patch is no longer necessary,
as its changes are already included.

Adds *all* code, rather than selecting just the code we use.

Other changes:
- README.android:
  - Update version
  - Android.mk -> Android.bp
  - Remove no longer relevant comment about applying the intel patch
  - Remove comment about deleting unused files
- README.version:
  - Update version and source link

Change-Id: Ib24cfcfec3e2f63eed8b3b9f0920b1018b30dd64