Remove ConnectivityManager and its usages from NetworkStatsService.

NSS needed it for getting VpnInfo[], NetworkState[] and
activeLinkProperties which it used to query via ConnectivityManager.

For VpnInfo[], this was racy as NSS may ignore intermediate changes to a
VPN's underlying networks. See http://b/123961098 for more context.

It may also lead to deadlocks b/w ConnectivityService and
NetworkStatsService. See http://b/126245192 for more info.

This change will ensure that NSS is never contending on any of
ConnectivityService locks.

This change also is cherry-picking cleanup made to NSS in
http://aosp/628368.

Bug: 123961098
Bug: 126245192
Bug: 120145746
Test: atest FrameworksNetTests
Change-Id: Ia687845888434c8ddd24bdf44b4c70dfe80e03f5
Merged-In: I57e117bb4e9efe491b19d6b5a479f2d58d1c58e6