[Android] Simplify the FolderBrowserAdapter a little, as well as the Fill method within FolderBrowser.java. Previously the fill method would create an entire new adapter and assign it to the backing ListView. This is pretty unnecessary, so what it now does is, when the function is called, it clears out the adapter, then simply fills it in again with the new directory's contents. Simple, and doesn't require a reference to the actual ListView to be used.