Change computeOutputColorType() to mimic old behavior

This will prevent behavior changes in BitmapFactory when
switching from SkImageDecoder to SkAndroidCodec.

We will only choose kGray8 if it is explicitly requested (and
also supported).

Additionally, we will always decode GIFS and WBMPS to
kIndex8.

BUG=skia:

Review URL: https://codereview.chromium.org/1519843002
1 file changed