[PiP on Desktop] Add PipDesktopState for PiP checks in Desktop Mode.

This is a refactoring work to provide a centralized place to check PiP
in Desktop Mode status.

Bug: 350475854
Test: atest DesktopTasksTransitionObserverTest DesktopRepositoryTest
DesktopTasksControllerTest PipSchedulerTest PipDesktopStateTest
Test: Manually verify all PiP on Desktop CUJs are WAI.
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I06114e0bb4194fefa8a38cc2b4fc6c94c47f3907