Show bouncer over dream.

This changelist allows the keyguard bouncer to be dragged up from the bottom of the screen over dreams. This interaction is accomplished by controlling the expansion of the keyguard view directly, allowing it to appear independently of the rest of the NotificationViewPanel.

Test: atest BouncerSwipeTouchHandlerTest
Bug: 211506329
Change-Id: I513d64e26b634b4e15bf1ba00af5e36e6df70eee