Prevent elements from jump-cutting after an interruption

This CL adds initial support for interruptions.

Before this CL, whenever a new transition was started, all elements
taking part in this transition (i.e. elements in the transition
fromScene or toScene) would instantly jump to their new state in this
transition.

This CL improves this by tracking the current transition used by an
element. If that transition is interrupted (i.e. the transition for that
element changed), we save the current state of the element. Then, the
next time we compute the state of the element, which is the first time
we compute the state of the element given the new transition, we also
compute and save the diff/delta between the state we saved earlier and
the new state. This delta is then animated to zero and added to the
state computed with the new transition. That way, we nicely animate to
the new state of the new transition while preventing jump cuts caused by
the interruption.

This CL adds support for elements offset, alpha and scale. The size will
be supported in a follow-up CL; it is quite harder to support given that
elements can be measured only once.

The unit test focus on offset only at the moment, because testing scale
and alpha in a unit test can't really be done without changing
production code. I *might* add some screenshot tests for this in another
CL.

See b/290930950#comment5 for details on how this works.

Bug: 290930950
Test: ElementTest
Test: Performed a lot of different transitions in a row in the STL demo
 manually.
Test: This change should not have any impact on usages outside of the
 STL demo given that interruptions were disabled everywhere in
 ag/26621600 and will be enabled later after thorough manual testing.

Flag: N/A

Change-Id: I5b2ee38d71b947cd3aaf587194281c75eca1a0ca