Do not cancel car volume dialog dismiss anim

A burst of ACTION_OUTSIDE events would cause the dialog
dismiss animation to repeatedly cancel and restart before
it could finish.

Add a flag to keep track of the dismissing state, and let
the animation finish once it has started.

Change made based on proposal from partner.

Bug: 142877093
Test: manual
Change-Id: I16a76bd7a65313027751f8f0e223d0346626c37b
(cherry picked from commit c8d3ef17af781731b558302efada5c1e7bebd096)