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: m
Flag: EXEMPT removing an unused API
Change-Id: Idb1fb7e7c0467d32be0ac851a0ba80171e206755
1 file changed