Split connectivity JNI and service jar

The VPN JNI code is moving to the tethering APEX with
ConnectivityService, so it needs to be split out of libandroid_servers.

Also move the service-connectivity.jar build rule to
packages/Connectivity together with the jni build rule.

Bug: 171540887
Test: m, device boots and VPN (L2TP and VpnService) verified working
Change-Id: Ic29096e2280ce928729315f53b2159b620da49d5