Prevent NotificationShelf from jump cutting notifications when appearing

Even with this change, there are still issues remaining to the overall sensitive reveal animation. I'll address those in subsequent CL's.

Bug: 243634602
Flag: SENSITIVE_REVEAL_ANIM
Test: Manual, i.e. observing sensitive reveal animation when face unlocking. Also checked that the shelf is still behaving normally for other scenarios
Change-Id: I237c6e96753b0333de7ef9a3a6ae4f430788c7c2