Enable multidex handling in lein-droid

With this patch, it is possible to build an application consisting of more than one dex File, by setting the --multi-dex option in :dex-opts.

Rathe than forcing dex to output a classes.dex file, this modification only passes the target directory as the output directory, which causes dex to automatically create classes2.dex, classes3.dex... etc. when the --multi-dex option is enabled.

Since ApkBuilder only provides constructors that only accept one dex file, the ApkBuilder is constructed without any dex file. Instead all files with a *.dex extension are retroactively added to the APK.

For details see
http://blog.osom.info/2014/10/multi-dex-to-rescue-from-infamous-65536.html