Fix DesktopPipTransitionObserver dependencies in WMShellModule.

The original commit 148cfd94 for PiP in
Desktop Mode caused a crash when the DESKTOP_WINDOWING_PIP flag is not
enabled due to the forced get() calls.

This CL is a fix forward along with the reland of the original CL.

Bug: 401865824
Bug: 401978578
Test: atest DesktopTasksControllerTest
DesktopTasksTransitionObserverTest
Test: Disable DESKTOP_WINDOWING_PIP flag and check that there are no
crashes and tests still pass
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I3615a8f1c30525182cea69aac819b192b7a9b499