Fix regression in webview exploit.

I moved some code around for the android webview exploit in #3418 and forgot to update a call from `js` to the `add_javascript_interface_exploit_js` method in the mixin I created, which breaks the primary vector (serving html to a vulnerable browser).

Looking at the code it is apparent that this fixes a bug, but to be sure I verified against a 4.1.2. target.

Verify:
- [ ] Run the `android/browser/webview_addjavascriptinterface`
- [ ] Get a session by navigating an android browser directly to the URL served by the exploit