Add Gray8 pixel config

Add Gray8 pixel config

This is still just linear (non-sRGB), but adding sRGB will
be the next step. I've verified that this is really making
R8 textures when uploading Gray8 bitmaps. Tests pass, and
the all_bitmap_configs GM still renders correctly (unlike
when we just mapped Gray8 to Alpha8).

This adds another pixel config, which could grow our cache
footprint, but the benefits of not using 4bpp for 1bpp data
should outweigh that?

BUG=skia:6110

Change-Id: I4fc4c2479fc25f1d278e174a9bb5b542a0cb184c
Reviewed-on: https://skia-review.googlesource.com/6817
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>