Enable both static and dynamically linked libpng

Enable both static and dynamically linked libpng

All platforms except android are configured to use the statically linked copy
of libpng. Android uses the system provided dynamic copy for SkImageDecoder
and the static copy for SkCodec. The exception being android framework builds
that currently use the dynamic copy everywhere.

This CL also enables NEON optimizations for libpng.

Review URL: https://codereview.chromium.org/1058823002