Stop using DesktopModeStatus in windowdecor and splitscreen

The two modules are two inter-dependent to be processed independently

I also had to correct a test that fails on devices using HSUM because of
the assumption the user running the test is the user 0.

Bug: 395863348
Flag: EXEMPT (refactor)
Test: atest WMShellUnitTests
Change-Id: I970997273960ecc605de2828c4d248b5bbf39017