AImageDecoder: allow no color conversion

Bug: 135133301
Test: I5e8bdcdae6837db23c0f4ef08f931f3bebe0ce0d

Previously the default SkColorSpace for AImageDecoder was set to the
result of SkAndroidCodec::computeOutputColorSpace. If the image has a
profile that does not map to an SkColorSpace, it will return either
DISPLAY_P3 or SRGB. Using that at decode time will result in color
conversion.

Instead, default to a null SkColorSpace for such a profile, resulting in
no color conversion. If the image has no profile, default to SRGB, as
usual.

A client that wants SRGB can still request and get that, but this allows
getting the raw pixels for an advanced client that may want to do its
own conversion.

Change-Id: I489f31fef79dec11e97c8e8fb9207adb77a3d0c7