Add transient views to ViewGroup

This change allows clients to add temporary (transient) views to a
list of such views in any layout container. These views will be drawn at
the specified index when the container is being rendered, but will otherwise
not participate in the normal events of child views, such as focus,
input, and accessibility. The purpose of this functionality is to
enable better animations when views have been removed, such that clients
can add them back into containers temporarily while an animation runs,
then remove them when the animation finishes. This functionality is similar,
albeit a subset, of the what ViewGroupOverlay provides, but this API
allows clients to interleave these views with the other children in
the container, which allows correct drawing order. ViewGroupOverlay,
and the older internal mechanism used by the old animation package,
draw all of their views after a container is drawn, which pops
these temporary views on top of the other children in the container.

Issue #18621099 Enable View overlay to respect elevation and shadows

Change-Id: I530df69b406aa27b9f551f5724384f4dd1215a6f