Propagate shared target values to ancestor STLs (1/2)

This CL improves support for shared values in nested STL by implementing
the same synchronization mechanism that is already done for elements:
nested STLs now propagates the target value of their shared values.

Similar to nested elements, this solution is not perfect and does not
nicely handle interruptions or multiple STLs transitioning at the same
time.

Bug: 438406908
Bug: 440007127
Fixes: 438406908
Test: AnimatedSharedAsStateTest
Flag: com.android.systemui.scene_container
Change-Id: I9087839df1f9808f3b2ecf1b764fdf2cf72ebbbf