Introduce a subject to assert changes over rectangles

In several flicker tests it is necessary to check if a window or layer position is moving or resizing.

To facilitate these checks this CL introduces RectSubject (accessible through WindowStateSubject and LayerSubject) to assert rectangle behaviors

Bug: 174238224
Test: atest FlickerTests WMShellFlickerTests
Change-Id: Id568fd9dbd59f176eeb84323c321553221932685