Make ImageDecoder#getSampledSize private

Bug: 152322291
Test: make

I originally added this thinking a client might use the following
pattern:

    Size size = decoder.getSampledSize(sampleSize);
    decoder.setTargetSize(size.getWidth(), size.getHeight());

But this is unnecessary. The client can directly call:

    decoder.setTargetSampleSize(sampleSize);

OTOH, if the client wanted to use a crop, too, then it would be useful
to know the exact sampled size. That could also be satisfied by adding
accessors that return the target size.

Change-Id: Id61907a8268f424d70503598d5bf844cde469d60