Fix flicker caused by disabled shared animations

This CL addresses a bug that was caused by ag/24901467: now that shared
element animation can be disabled, Element.last{Size,Offset,Alpha} could
be set by 2 different scenes at the same time, making this value
unpredictable.

This CL fixes it by also storing the last size/offset/alpha in a given
scene. When defined, this value will be used instead of the last shared
value. When undefined, we use the last shared value.

Bug: 291053742
Test: Manual, see video at b/291053742#comment3
Change-Id: Iebf2827603ec583fbc99348a24620e3d2026a179