Embed layout padding in nine patch images

- Added a new custom PNG chunk that carries the layout padding ints.
- Extract the padding ticks from .9.png images and store in the chunk.
- Load the padding information at runtime into Bitmap and NinePatchDrawable.

- The new chunk is ordered first so that it doesn't cause a problem in older
  versions of the platform.

Bug: 6087201

Change-Id: I5de46167a1d44b3ec21065b0c165e594b1dc8399