Annotate Stable classes/interfaces to optimize compositions

This CL annotates most of the public STL classes as @Stable (which they
are) so that we don't recompose when it's not necessary. This CL adds a
small test to ensure that existing elements don't recompose when
starting or finishing a transition.

Bug: 291071158
Flag: NA
Test: ElementTest
Change-Id: Idccc990000d7678261c6a14c7a86aedbc353221d