Stop accessing VPNs in getAllVpnInfo.

This is only used for NetworkStatsService and only called on the
handler thread, so it can be replaced by a simple scan over
mNetworkAgentInfos without having to take any locks.

Bug: 173331190
Test: passes existing tests in ConnectivityServiceTest
Change-Id: I194e0cc55603a0f59f7138f38329f505b55da132