Extract movable contents outside of the Element class

This CL moves the Maps and objects that are used to move the content of
movable elements outside of the Element class. That way, composing a
MovableElement won't require the Element object, which will allow to
remove the last calls to Snapshot.withoutReadObservation {} and map
mutations during composition inside Modifier.element().

Test: MovableElementTest
Bug: 291071158
Flag: N/A
Change-Id: I5cd94d6685d8640650ccfd28bdbd2de1867a5373