Only treat "null" bounds as fullscreen

When long pressing on the recents button, we made it one pixel smaller
than fullscreen so we don't dismiss the stack immediately again.
However, this is a huge hack, and lead to problems with navigation bar
background because there we actually rely on the fact whether
the window is fullscreen or not to determine whether to draw the
navigation bar background, which lead to flickering.

Bug: 26777526
Change-Id: Ifdfcf3ad4138bc88c5164177cd20f1ff1635085f