fix a memory leak and memory corruption in RefBase

we would leak a weakref_impl if a RefBase was never incWeak()'ed.
there was also a dangling pointer that would cause memory corruption
and double-delete when a custom destroyer was used to delay the
execution of ~RefBase.

it turns out that the custom destroyer feature caused most of the
problems, so it's now gone. The only client was SurfaceFlinger
who now handles things on its own.

RefBase is essentially back its "gingerbread" state, but the
code was slightly cleaned-up.

Bug: 5151207, 5084978
Change-Id: Id6ef1d707f96d96366f75068f77b30e0ce2722a5