Merge "Upgrade libpng to v1.6.38" am: 1640fa9077

Original change: https://android-review.googlesource.com/c/platform/external/libpng/+/2305386

Change-Id: I2b5bf136b55627fc010bae198b937402b202a18c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>