Fix scaling + cropping in ImageDecoder

Bug: 141735646
Test: I9ab9115ee4cbc74549f7b914d99b667c700199d5

When using a subset, do not change desiredWidth/desiredHeight, which
changes the scale used below. Instead, just change the size of the
SkBitmap, so it looks correct.

Change-Id: Ie9682afb9324bcdfc81f1d9664f9fcc3891f8939