[Mobile, Android] Store cached avatars in cache dir instead of app data dir

Android allows each app to have a cache directory where it can store files separately from persistent the app data dir; this dir can then be automatically emptied by the system or manually emptied by the user without also destroying persistent data.

Need to check whether Ti.Filesystem.tempDirectory returns the cache dir or an even more temporary dir; might need to poke something to get at the official cache dir.