Fix leakage of DisplayContent referenced from TransitionController

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.
(cherry picked from commit 491914c1)
(cherry picked from commit c3eaf662)
Merged-In: I44e2a794981fc114ada0ce98cd2471adf623d856
Change-Id: I44e2a794981fc114ada0ce98cd2471adf623d856