Do not allow panel expansion with bouncer over dream.

This change prevents the PanelViewController, the parent class of
NotificationPanelView, from expanding on end motion when the bouncer is
shown over dream. The BouncerSwipeTouchHandler now informs
CentralSurfaces when the bouncer is showing over the dream. This is
independent of the normal signal for bouncer showing. This prevents the
panel from appearing erroneously on motion cancel during swipe down on
the bouncer. This cancel comes as a result of the bouncer pilfering the
touch events once the gesture has been detected.

Fixed: 228347831
Test: atest BouncerSwipeTouchHandlerTest#testInformBouncerShowingOnExpand
Test: atest BouncerSwipeTouchHandlerTest#testInformBouncerShowingOnCollapse
Change-Id: I83a599baea904c8b1a2cea3a7c55dbe9c166c3d2