Notification panel on tablet does not handle back and home key events.

1. Notification panel on tablet does not handle back and home key events and
   as a result the notification panel stays open. Hence, after opening the
   notifications panel, pressing the back key on a keyboard will move back
   in the app instead closing the panel. Same happens for the home key.
   The expected behavior is if the panel is open the back button will dismiss
   it and the key should be consumed by the panel. The home key should hide
   the panel and the key should not be consumed by the panel so the system
   can do the right thing.

bug:6902903

Change-Id: I06e8ceea1f51b998e6703d70dcb3a24128d5a581