Use the path returned by 'getExternalFilesDir(String type)'
From the documentation:
"Returns the absolute path to the directory on the primary external filesystem (that is somewhere on Environment.getExternalStorageDirectory()) where the application can place persistent files it owns. These files are internal to the applications, and not typically visible to the user as media.
This is like getFilesDir() in that these files will be deleted when the application is uninstalled, however there are some important differences:
External files are not always available: they will disappear if the user mounts the external storage on a computer or removes it. See the APIs on Environment for information in the storage state.There is no security enforced with these files. For example, any application holding WRITE_EXTERNAL_STORAGE can write to these files."
For further details: http://developer.android.com/reference/android/content/Context.html#getExternalFilesDir(java.lang.String)
So yes, you can use external storage and have the files deleted when the app is uninstalled (as long as you are happy with the lack of security).