res_stasis: Fix dial bridge unload.

If the dial bridge has been created it must be released by calling
ast_bridge_destroy, simply releasing the ao2 reference is not enough.

Also move stasis_app_control_shutdown earlier in unload to ensure the
bridge cannot be created or grabbed after the app_bridges container is
released.

Change-Id: I372302de94ca63876069e2585a049c5060e5e767