Changes

Refactor xua_layer_manager object

Several changes to improve separation of the LM object and its
lifecycle:
* Store in ss7_asp a pointer to a xua_layer_manager, which contains all
  implementation dependent information inside its priv pointer.
* Add a new free_func field to the xua_layer_manager struct in order to
  be able to free it without knowing how it was allocated.
* Get rid og the loglevel param, LOGL_DEBUG was always passed and that
  was actually an implementation specific detail of the default xua
  layer manager.
* Pass pointer to function to be called by LM to submit primitives to
  M3UA.

Change-Id: Ia96ebf40444f46ad718d61befbecb523f267fd6c