Add test coverage for NetworkAgent callbacks.

Tests the onNetworkCreated, onNetworkUnwanted and
onNetworkDisconnected callbacks, and the teardown delay timer.

Bug: 181941583
Test: atest --rerun-until-failure 500 ConnectivityServiceTest#testNetworkAgentCallbacks
Change-Id: If539cf5d01ba23193afab2433ed0ac4e7f0550ec