Remove IWindowManager#setShouldShowSystemDecors

With the introduction of content mode,
DisplayWindowSettings.setShouldShowSystemDecorations should only be set
within WM when Display.canHostTasks() is initialized or changes. This CL
removes IWindowManager#setShouldShowSystemDecors() to prevent its
exposure outside of WM.

Bug: 400864452
Test: ActivityTaskManagerServiceTests
Test: MultiDisplaySecurityTests
Flag: EXEMPT removing an unused API
Change-Id: If89c8931a94471b5cb877fea49f55a78f338893c