resolv: Register multiple metrics event listeners

Currently, Netd unit test can't verify metrics event content because framework
is the only one metrics event listener of Netd. Add a support that Netd allows
multiple metrics event listeners registration. Netd unit test could verify
event content via registering its event listener.

Test: built, flashed, booted
      system/netd/tests/runtests.sh passes

Change-Id: I6f7f3240aa23cadcda59e037d7a027136525bc97