Use TestableLooper.RunWithLooper in unit tests

These tests must run on a looper thread because they're UI related.
TestableLooper takes care of running the class/tests on a looper thread
without manually posting and awaiting a CountDownLatch.

Bug: None
Test: atest DesktopModeWindowDecorViewModelTests
Change-Id: I37127a0f98c25d52f75746f4915e6cba552838f6