Real ActionBar part II

- Implement an ActionBarDrawerToggle, and wire it to the ActionBar and nav
  drawer.
- Make the ActionBar hide/show upon scrolling far enough in the WebView.
- Update the hamburger drawable, and also use an xml drawable for the
  ActionBar background, so that we don't need a 9-patch png.

Change-Id: I6060e6acbdbbe6e7f88e1ce247cfb9c964be322e