Introduce element transformations across NestedSTLs

A STL containing NestedSTLs can now reference all elements of any
nestingDepth to apply transformations.

There are a few limitations:
- There is currently no mechanism to select priority over which STL
transition is chosen when they run in parallel. Instead the priority is
currently always the STL with the lowest nestingDepth. This means that
if a child runs a transition and an ancestor starts a transition that
has a transformation for the same element defined (only then) the
element will adhere to the transformation of the ancestor.
- Interruptions may not always work as expected. For example when the
child completes a transition which ends in a scene where the element is
not present anymore, then the ancestor transition will still end but
then the element might suddenly disappear (because it's not composed in
the child STL anymore)

Bug: 376659778
Test: new unit tests and manual test app
Flag: com.android.systemui.scene_container
Change-Id: I30a025bd749ed0e1b0b400b51532cba748456bc7