Fix issue with fixed toolbar

This MR fixes #183 (closed). The solution was simple, I re-implemented the showAsFixed method of ext/BrowserToolbar.kt' and use it to set engineView.setDynamicToolbarMaxHeight()andengineView.asView().translationY` to the correct values depending on the toolbar position.

The easiest way to test that this fix works is to log into Twitter and check that you can see the website's bottom navigation bar, both when the Ceno toolbar is at the bottom and top. I can't really think of how to implement an automated test for this, since it is dependent on web content layout.