Mark children as detached instead of actually detaching them.

When detachChildren is called, mark children as detached instead of removing
them from the client. This is so the children can be re-attached again when
they're reparented. Check if layer is detached before copying currentState
to drawingState so detached layers aren't updated. Also check if layer
is detached before removing the layer since they should no longer be
reachable if they're detached.

Test: DetachChildrenThenAttach
Bug: 111297488
Change-Id: I31b9eb1398f02e9d68dd3f7a1f231e1cef9fb0f8