Document behavior and usage of ViewGroup.bringChildToFront()

A call to ViewGroup.bringChildToFront() or View.bringToFront()
(which delegates to the parent's bringChildToFront() method) needs
to be followed by a call to requestLayout() and invalidate() on the
parent container in order for the changes to
actually happen. That is, the order of the child views would change, but
the parent container would not run layout or even invalidation without
being told to, so there would be no visible change until something else
caused a layout and invalidation to occur.

This change clarifies this requirement in the javadocs.

Issue #8667065 bringtoTop does not work

Change-Id: Ibe41a6318dddf9fb79382e1c9fd1d21ab4510976