SurfaceControlViewHost: Release ViewRoot from finalizer

If we don't explicitly go through the doDie process the
WindowManagerGlobal instance will hold the ViewRoot alive
indefinitely. This accidental leak could be very expensive so
we prevent it with a finalizer.

Bug: 157709599
Test: Existing tests pass
Change-Id: I4fdf368eed4b4e43faacd9f62b6d9fddfd9b7ef2