When updating notifications, always reuse the last row.

Ordinarily, changing the "when" slot when updating a
Notification will move its expanded payload to the bottom of
the notification panel's list. But if it's already at the
bottom, this causes an unsightly out-then-in animation
sequence. We now check for that exact situation.

Bug: 3222274
Bug: 3162218
Change-Id: I8b643b3f8bce64e57a22bdbb06d92c196c2484c2