fix a surface leak in SurfaceFlinger

SF kept a strong reference to ISurface until the
window manager removed the surface from the screen.
This fell appart when running standalone tests, that is
when the window manager wasn't involved.

When the window manager is around, it would clean-up surfaces
even when an application died.

with this change, SF is able to do its own cleanup without
relying on the window manager.

the change is very simple, we simply don't keep a reference
to ISurface and make sure no more than one of them can
be created.

Change-Id: I61f2d7473bf8d4aa651549a846c34cdbb0d0c85a