Add plugin interfaces to control notification menu / swipe behavior

1) Updates the notification menu plugin to do more:
- The plugin can now supply the view it wants to display behind a
  notification as the menu
- Touch interaction (i.e. notification translating on top) is piped
  through to the plugin so it can update the view behind the notification

2) Makes NotificationMenuRow an implementor of the plugin which moves
   a bunch of that logic out of NotificationStackScroller

Test: runtest systemui
Change-Id: I8723b96c195ff7311317d5523418cfcbe2a79b52