Allow swiping down notification shade over dream.

This changelist enables swiping down the notification shade over the
dream. NotificationShadeTouchHandler tracks swipes that originate from
the status bar area of the dream and redirects these motion events to
the NotificationPanelViewController.

Test: atest NotificationShadeTouchHandlerTest
Bug: 267565290
Change-Id: I72d54cf33601dacc0a2a6adfd2b7639615061c09