Fix issue where apps could prevent the user from going home.

Now we have a 5-second time after home is pressed, during which
only the home app (and the status bar) can switch to another app.
After that time, any start activity requests that occurred will
be executed, to allow things like alarms to be displayed.  Also
if during that time the user launches another app, the pending
starts will be executed without resuming their activities and
the one they started placed at the top and executed.