Widgets tray should preserve scroll position during screen rotation

> This used to work, but when I got rid of preloading which was
handled by linear manager to increase scrolling speed, the save
and restore instance that was being handled by the linear layout
manager created using Launcher context also went away. Hence,
bringing back custom LinearLayoutManager.

Change-Id: Ibccdf80533a5ace06cc6c6932b257c223cbac0ff