AImageDecoder: update comments around dataspaces

Bug: 135133301
Test: I5e8bdcdae6837db23c0f4ef08f931f3bebe0ce0d

A companion change (I489f31fef79dec11e97c8e8fb9207adb77a3d0c7) makes
AImageDecoder default to doing no color conversion. Update comments to
reflect that.

Change-Id: Icc3b0f101598ce98c458ff43597ee05b3662664c
1 file changed