Fix inSampleSize != 1 gainmap decoding

When decoding with a sample size we cannot use the width/height of
the subset to allocate the gainmap bitmap as that will be scaled
by the sample size.

Instead split out the destination width/height from the decode
subset to avoid trying to pack too many meanings into a single rect

Fixes: 289103338

Test: manual inspection of inSampleSize 1, 2, and 3 with & without
inBitmap being specified

Change-Id: I0685579d6835a6a06d73ef50c59b08611ca80211