CE: Unit test coverage for Output::prepare and Output::rebuildLayerStack

Also modifies ui::Transform(int) to take two extra optional parameters
to set the width/height to use (forwarded to ui::Transform::set())

Bug: 144116499
Test: atest libcompositionengine_test
Change-Id: Ifb972c2619af41cb3999c50c082bce20b422c1ea