Even though the categoyr mage loader explicitly says not to cache
images on disk (because they are not coming from the network anyway),
UIL still uses the `FilenameGenerator` to come up with a disk cache name.
Because the file name generator takes the "path" of the URL being
downloaded, and the categories are loaded like "drawable://2134234",
there is no path. As such, the file name ends up being meaningless.
This results in the image loader testing for the existance of the file
on disk (even though we asked not to cache on disk), and then failing
with an IOException (that gets swallowed).
By providing a meaningful name from the file name generator, it now
works as expected.
Fixes#1039.
be727ae7c0