Made preview in Android XML editor work as expected

First of all, thanks a lot for the code, just what I needed :)

I've made a small change to the StickyListHeadersListViewWrapper and the StickyListHeadersListView so the list can be displayed in the preview window of Android's XML editor.

Without the change a crash occurs because there is no Context and because the list adapter used for the preview does not implement the required interface.

The classes now use "isInEditMode()" to detect when the list is shown in the editor's preview window and react accordingly.