Simplify PMColor preprocessor logic, move it all into SkColorPriv.h

This was spread across SkColorPriv.h and SkColorData.h, and was
accomodating fictional users that set SK_PMCOLOR_IS_* themselves.

Also, remove an unused define and stop grilling colors.

Bug: skia:
Change-Id: I70efd51052f32afdc3e19fd55dd38270d98b6b76
Reviewed-on: https://skia-review.googlesource.com/c/167392
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed