Release SC when RootTaskDA is no longer organized

WM Shell receives a leash for the DA that it wants to organize. However,
it's not releasing the SC when it receives a vanished callback. This
causes the SC to leak every time a new RootTaskDA is organized and then
vanishes.

Test: Add and remove simulated displays. Ensure no leaks
Bug: 231752353
Change-Id: I181695f4bae65ccc0cd25e1b5480e33307e7834d