Use DesktopModeWindowDecorViewModel when providing WindowDecorViewModel

Before this CL we create two instances of
DesktopModeWindowDecorViewModel since we provide one
WindowDecorViewModel and one DesktopModeWindowDecorViewModel.
With this CL we reuse the DesktopModeWindowDecorViewModel instance for
the WindowDecorViewModel provider.

Bug: 379672644
Test: NA
Flag: EXEMPT refactoring
Change-Id: I74a7a98e70f7613b86a8bd14075c994d4e3c793c