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.
Change-Id: I44e2a794981fc114ada0ce98cd2471adf623d856