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
libpng: Fix for memory leak while indexing the PNG images

de-allocating the index resorces at proper place. png_read_destroy()
making png_ptr->index NULL and due to this de-allocation was not
happening even though code exist (it was like dead-code).

Change-Id: I770bbfc486365b1a8b6a8dc5f128aa2b3a34cd50
CRs-Fixed: 551617