Merge "Upgrade libpng to v1.6.38" am: 1640fa9077 am: 3f67af02ac am: 1a56ed26ad

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

Change-Id: Ia0b22bce775aab319c61e7f43d7a186033aa4b34
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>