Refactor back nav logic to its own class.

Then wire the class into FreeformTaskTransitionObserver. This guarantees
that the changes will be triggered before DesktopTaskChangeListener
updates DesktopRepository.

Bug: 410540195
Bug: 407480909
Flag: EXEMPT Refactor
Test: Existing unit tests are passing, manually verify that back
navigation works fine.

Change-Id: I983145a78417572169abaca04b4c69c8b844883a