Introduce SharedElement transitions with NestedSTLs

This is a first iteration to support shared elements across nestedSTLs.
This version has several limitations that may be addressed in future
CLs.

The main mechanism to drive this is by sharing the elements map and the
lookaheadScope of all nestedSTLs. This means that they also share
Element.stateByContent. Each child writes a reference of its state to
its own content but also one into each contentKey of where it is
composed in its parent. Before `stateByContent` would hold a state
for each content that would compose an element, after this change
it also holds a state when any of the nestedSTLs compose this element.
This means that always the STL with the lowest nestingDepth has to
render the element as the element is not actually placed in the
other scene of that transition.

Known limitations are:
- Interruptions are not supported (will jump cut)
- Side-by-side NestedSTLs are not supported. That means if we have a
ParentSTL that contains NestedSTL1 and NestedSTL2 then a shared element
can be transitioned between the ParentSTL and the NestedSTLs but not
between NestedSTL1 and NestedSTL2. Multiple nesting layers are supported
though, so ParentSTL > NestedSTL1 > NestedNestedSTL1 would support
sharing between any of them.
- If the sharedElement appears more than two times on the screen
(due to multiple transitions of multiple STLs transitioning at the same
time) the behavior is not supported.
- A sharedElement can't be present twice in the same hierachy. Meaning
that ParentSTL sceneA contains an element. There can't be a NestedSTL
on the same scene that would contain the same element in any of its
scenes. This would result in a conflict of the idle state.
- Currently the sharedElement is always rendered in the ParentSTL during
transitions. We don't support more control for this behavior as we do
for normal sharedElements yet.
- If you disable a sharedElement transition for a specific transition
you can only reference the element in the parentSTL to define a
custom transition. This is a general limitation where you can't
reference elements within NestedSTLs in transition DSL.
- NestedSTls are not allowed to share the same contentkeys

Test: Manual test app added + Units tests added
Bug: b/376659778
Flag: NONE not in production yet
Change-Id: I0478d4f73f189e506ace39f2b3544ea6acc75ead