Fix b/12348405. Return null drawable.

The initial implementation of getDrawable always
returned a drawable. However, the PhotoViewFragment
used the null state for various state decisions.

This made images that weren't on device in Gmail not
show up and no downloading progress was shown. Now this
happens. Hooray.

Change-Id: I55df738f55353d9c4b599c2e497ca8ed4768c059
3 files changed