Provide a shared library version of libpng for on-device linking.

Bug: 9593270

This change also switches pngtest to use the actual shared library (and not
recompile all of the files another time as one giant blob).

Change-Id: I57ce13de31dd4973f47896cd5d056badf4a8c4db
Merge https://android.googlesource.com/platform/external/libpng into kk-devel

Conflicts:
	Android.mk