(A)ImageDecoder: fix sampled dimensions with exif

Test: I138ec784a77253c7ae94765d0670e5947d97caf5

Recently (Ib93b0ced09fa3cca4a6681745406355c48158fae), handling exif
moved from SkAndroidCodec to hwui/ImageDecoder. This missed
getSampledDimensions, which reports the sampled dimensions without
taking exif into account. Fix this for both
android.graphics.ImageDecoder and AImageDecoder. Note that in the Java
case, the method is private, and although the method was returning
swapped dimensions in some cases, it gets corrected by
getTargetDimension, so there is no user visible change.

Change-Id: I918328c39d6230ae6ba4cab0733fff0732b39888