Get rid of the global actions menu on tablets.

We have the system tray for that stuff.  Make long press on power
go directly to the shut down confirmation dialog.  Make that dialog
auto dismiss when the screen goes off.  Reword it when we're skipping
the global actions menu, because it feels too abrupt to say "The
phone will shut down" when they haven't clearly pressed a button
that says shut down.

Change-Id: I8c92a1362bd4ed242e6cd94c2dae6d9ff9c4c34d