Make (Movable)ElementScope extend ElementBoxScope

This CL exposes the BoxScope of Element and MovableElement so that we
can easily create an Element that wraps its content but that have a
background that is exactly the same size of the element.

Test: MovableElementTest
Bug: 291071158
Flag: N/A
Change-Id: Iee96ece7b792c8e78a3c1dd71726e5317fc111d1
4 files changed