Rename SkImage_Generator to SkImage_Lazy

Rename SkImage_Generator to SkImage_Lazy

This removes a long-standing source of confusion: SkImage_Generator was an
image that wrapped an SkImageGenerator (with an SkImageCacherator stuck in
the middle). We could choose to rename either one, but SkImageGenerator is
public, so take the easy road and rename the private image subclass. Given
the existence of SkImage::isLazyGenerated, this name seems appropriate.

Bug: skia:
Change-Id: I061ece94f48538efb1dc5548010f6ca7d438a69b
Reviewed-on: https://skia-review.googlesource.com/13979
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>