fix path in expansionFile.getAssetFileDescriptor

First, thank you for this plugin :)

In the reader, the expansion filename is repeated
expansionFile.getAssetFileDescriptor(fileName + "/" + filename);
This returns a path like main.1.com.company.appname/image.jpg. This is
working if we have zipped a directory called main.1.com.company.appname.

But, in many cases, content is added directly to the zip, with no deps
on the expansion file name.
We should use only the filename
expansionFile.getAssetFileDescriptor(filename); to return only path
image.jpg
This corresponds to a cleaner way to get the content.
If your zip file contains a directory called main.1.com.company.appname
you should now use it like that
XAPKReader.get('main.1.com.company.appname/image.jpg’, function(url) {
…