Use kRGB_888x_SkColorType when importing R8G8B8X8 AHB.

Bug: skia:
Change-Id: If32fa0c711dd33c78cd57c73475f74bdeb470d8b
Reviewed-on: https://skia-review.googlesource.com/c/192420
Reviewed-by: Stan Iliev <stani@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>