Do not return Index8 from SkAndroidCodec::computeOutputColorType

Given that this is the only known use of Index8 color type,
this is essentially an experimental delete.

Bug: skia:6620
Change-Id: Ib363d237e0217f6e7f461a62e54d32892c428095
Reviewed-on: https://skia-review.googlesource.com/10586
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
2 files changed