Store an SkImageInfo on Bitmap

Color type, alpha type, and color space are being removed from
SkPixelRef.

The Bitmap class wants these to be mutable anyway.

For other uses of SkPixelRef, it is often convenient to reinterpret
the swizzle, alpha type, or color space of the pixel ref.

Test: Refactor
skbug.com/6535

Change-Id: Ie49ac64b46e171974a53614e1803ab18300e634e