Expose notification guts on menu shown

This change adds a few methods to NotificationMenuRowPlugin so that a
menu row can tell the stack scroller to expose the notification guts
when the menu is fully exposed.

TODO: fix the menu close animation so it happens after the guts open,
and if you close the guts by swiping you can trigger a flicker.

Also there are some things to make lint happy and lots of null checks.

Bug: 127998765
Test: swipe RTL on any notification
Change-Id: I46f7c7dc90032a1914f7b5513bc57e089887c722