Use latest display ID in DesktopModeWindowDecorViewModel

DesktopModeWindowDecorViewModel#mDisplayId is never updated after
it's initialized, thus once an app moves between displays and this
is used, the desktop mode state gets broken.

This bug was found as part of restart mene developement but not
directly relevant to the feature.

Flag: EXEMPT trivial bug fix
Bug: 421048151
Test: DesktopModeWindowDecorViewModelTests
Change-Id: I58ea1c398fd1478040f279126dd9fdd2806fa78a