Add immersive state to desktop repository

Changes the app header maximize icon based on whether the task is in
immersive state or not.

Flag: com.android.window.flags.enable_fully_immersive_in_desktop
Bug: 369402633
Test: atest WMShellUnitTests
Change-Id: Ic4f73212d34d07f14057c0475ecab99b3b22820a