Unlike AppTransition which is associated DisplayContent, the
TransitionController is a global instance. So it needs to unregister
the listener when the display is removed.
Bug: 214991741
Test: Start/stop screen recording multiple times (create virtual
display). The heap dump of system server should not contain
many DisplayContent instances.
Change-Id: I44e2a794981fc114ada0ce98cd2471adf623d856
(cherry picked from commit 491914c17c)
Merged-In:I44e2a794981fc114ada0ce98cd2471adf623d856
c3eaf66225