Bug 1039984 - [System] Trying to open a smartcollection after killing the Homescreen do nothing.

Allow Homescreen application window to be killed as other windows. But
avoiding to be marked as _killed as, from a system app perspective, the
homescreen is never killed.
Merge pull request #21862 from lodr/bug-1039984-smartcollections-and-homescreen

Bug 1039984 - [System] Trying to open a smartcollection after killing th...