Properly place the clear RTP receivers handler.
Instead of doing directly from the destructor,
do it on state change (onBeforeDestroy) via
the down to up virtual function realization search.
Is triggered by `AmSession::finalize()`.
Closes Coverity CID 642748.
Change-Id: I4c224425e9fbecedef3d29a6658f1723313b19c4