Add overlays to views

It is useful, particularly in animations, to be able to add a view, or at
least some graphics, on top of a view. For example, to have a child of a layout
fade away, we might want to remove the child from that layout and then fade it out
gradually. Meanwhile, we have to have a place to put that view where it will be
drawn. We could do this in the content container sometimes, but this is not a
reliable workaround in the general case, and may obscure other siblings/parents of
the layout/view in the hierarchy. A better approach would be to place a view/graphic
temporarily in the layout itself.

This feature adds the ability to add one or more Views and Drawables to an "overlay"
layer, after which the view will handle drawing that extra content when it redraws itself.

Issue #8350510 Add APIs needed for future animation capabilities

Change-Id: I70bf78c46ee3db8bd87ea1cdc2ecb5c0747ccbf9

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