AAPT2: Refactor PngCrunching

PngCrunching now has a slightly better heuristic of choosing to encode
an image as a palette or RGB. For small images, RGB compresses much better
than a palette.

The original PNG is used as-is (minus some optional chunks being stripped)
if the resulting crunched PNG is larger than the original.

9-patch handling is abstracted away from PNGs, paving the way
for other 9-patches, like WebP.

TODO: handle PNGs with 9-patch chunks already present, which
should just be passed through. This will allow for 3rd party
tools to generate 9-patches.

TODO: implement cheap transparency: when one color is used to represent
transparent, and all other colors are opaque.

Bug:30053276
Change-Id: I5167f53b91d1efa462d9f03d6b9108d9b541c0c1