Split window decor lifecycle to a TransitionObserver

There are several transitions involving window decorations that are
handled by Launcher, but we still need to tie window decorations to the
lifecycle of such transitions to release window decorations at the right
time. Thus we move this part of logic into a new class.

Bug: 241975249
Test: atest FreeformTaskTransitionObserverTest
Change-Id: Iff14be6f1fcb0b8ee7f634d5d41567348fff5c06