Add support for dialogs in Expandable (1/2)

This CL adds support for dialog animation in Expandable. It does so by
refactoring DialogLaunchAnimator so that its implementation doesn't
assume it is expanding a View anymore. For consistency with
ActivityLaunchAnimator.Controller, I introduced a
DialogLaunchAnimator.Controller to abstract the source of a dialog
animation.

This CL also slightly changes the behavior when a chain of dialogs is
dismissed (using DialogLaunchAnimator.dismissStack()): the top-level
dialog is not animated into the first source of the chain anymore.
The current behavior actually led to sub-optimal animations where it is
used (user switcher), so the new behavior from this CL is subjectively
better.

See b/230830644#comment11 for videos and ag/20139983 for an example.

Bug: 230830644
Test: Manual
Change-Id: I59cfc5e19d4de8e5d1fec962028d043cbd2c417b