Reduce hardware render sizes for ExpandableNotificationRow.

Do not create a hardware layer for entire notifications during shade expand/collapse; instead just layer the elements with actually overlapping rendering.

Bug: 191969252
Test: lots of shade expansion; use of perfetto to validate phone reaches rendering deadlines more often.
Merged-In: I86b626fb61c5844b968be33d982e418bf8938de8
Change-Id: I86b626fb61c5844b968be33d982e418bf8938de8