Add child sensitivity change listener to NotificationStackScrollLayout

With this change, the animation has still issues when the notification resize causes the shelf to appear and/or notifications to disappear. This is addressed in subsequent CL's.

Bug: 243634602
Flag: SENSITIVE_REVEAL_ANIM
Test: Manual, i.e. checking sensitive reveal animation when face unlocking.
Change-Id: I9f563a973e72c2789ee1cb3a4f26e361be9df411