Increase specificity of GrColorType computed for YUV planes (take 2)

When a single channel texture is used for a YUV channel we will
interpret it as kGray_8. When a single channel texture is used
as an A channel we will interpret it as kAlpha_8.

Change-Id: I0a02b849d214f63369dedf09b78195a293adc78e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/232142
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
14 files changed