Fix CustomContent/CMHome bugs with settings.

1. If Search Panel / CMHome is enabled, it would become visible
   in overview mode after any setting is toggled. Fixed.
2. If all icons on the last home screen were removed and Search Panel /
   CMHome is enabled, CMHome would become the only workspace, breaking
   most things. Fixed.

Change-Id: I5a31e8058f71922c2790c6ac43b09ed7bcbe768a