Collapse toolbar when hosting multiple fragments

Registers a FragmentLifecycleCallbacks to collapse the toolbar when an
activity is hosting multiple fragments, as indicated by a non-empty back
stack in the FragmentManager. This behavior is applied only when using
the expressive theme.

Bug: 438375587
Test: manual test
Flag: EXEMPT bugfix to specific case
Change-Id: I4b9c494872e1634b516fd99672cba4d742d40d2a