Fixing workspace state issue with option menu.

- Hitting a ctrl key combo will actually call onPrepareOptionsMenu(), which
  will change the state of the workspace incorrectly.  Instead, listen for
  the menu button, and only allow the user to enter overview mode if they
  are currently not dragging or in an overlay state (like all apps or
  widgets).
- Also making the overview buttons focusable so that they are focused when
  you go into overview mode.

Bug: 22483367
Change-Id: Ie6e9febd8a5a4e7ad25d745639d42d1c7b9801b4