Convert INetdEventListener to stable AIDL.

1. Add a new netd_event_listener_interface aidl_interface target.
   This interface is defined by netd and implemented by the
   framework.
2. Delete the netd_metrics_aidl filegroup and use the AIDL
   interface everywhere in netd.

Test: builds, boots
Test: system/netd/tests/runtests.sh
Change-Id: I70f2d51183dcbde03db932b66f44c2cb5579e311