Stop using VPNs in getActiveNetworkForUidInternal.

After this CL, ConnectivityService no longer needs to grab the
VPN lock and access Vpn objects to determine connectivity state.

Also make a synchronized (mVpns) block smaller.

Bug: 173331190
Test: existing tests in ConnectivityServiceTest
Change-Id: I3297b3ab8e75bdf4902c62dd7e492c3634cfc51f