Dismiss immediately to maintain consistent state.

Fix bug introduced by deferring nulling of mParent.

In dismissDialog the removal was being put on a queue while the
state of the Dialog was being updated immediately. This meant that
if a show() was called before the remove was executed it would try
and add the DecorView a second time. Boom!

Fixes bug 9370301.

Change-Id: I576d1e207c786bc2e21dfd40cb94f2b63a020fe2