[wm] Explicitly release surfacecontrol in mirrorDisplay

Don't rely on GC to release the surfacecontrol. If we stress
the API like in the case of tests, we may end up not freeing
resources in time and crash the system.

Test: test magnifier
Bug: 279622227

Change-Id: Id70c9387bce0d0e92d40cccdeaab232b9c61d7e8