Pixel conversion refactors: use raster pipeline for 565 and gray

Pixel conversion refactors: use raster pipeline for 565 and gray

I'm trying not to do too much in one CL.  But, in general, I hope
to drop (non-performance important/optimized) special cases and
use the pipeline.

BUG=skia:

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: I724d3982f1467f6232371360b860484f13b1ede8
Reviewed-on: https://skia-review.googlesource.com/8271
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>