Bug 1082885 - Make IME Menu SHB aware

wow, calling an internal method from an integration test is a line I'd like not to cross.
I get that focusing an input, dragging the utility tray down and tap the IME switcher will be pretty cumbersome, but maybe we can dispatch a `mozChromeEvent` instead [1]. 

[1] https://github.com/mozilla-b2g/gaia/blob/8a7d68b7d237a444e32825050474b20d99745424/apps/system/js/keyboard_manager.js#L312