NetworkStatsFactory: Take VPNs into account for network/battery stats

This change fixes detailed UID stats to ensure network and battery stats
both take VPNs into account. NetworkStatsFactory is being made aware of
VPNs enabled, and the full set of underlying networks present.

Since traffic can only be migrated over a NetworkStats delta, NSF
maintains a NetworkStats snapshot across all UIDs/ifaces/tags.

This snapshot gets updated whenever NSF records a new snapshot
(based on various hooks such as VPN updating its underlying networks,
network getting lost, etc.), or NetworkStatsService's
getDetailedUidStats() method being called.

This change widens the scope of the existing mPersistentSnapshot lock,
renaming it to mPersistentDataLock, and ensures that TUN migrations are
not done in parallel. Additionally, mVpnInfos is updated via
pointer-swapping, to reduce the scope of the mPersistentDataLock.

The safety of this change is predicated on:
1. NetworkStatsFactory lock not held, so services cannot deadlock through
the cyclical lock.

2. The broadening of the scope of the lock in NetworkStatsFactory has no
threading implications, as it is always the last (leaf node) lock held,
and therefore is impossible to have lock inversion.

Additionally, to ensure VPNs work with 464xlat, the VPN info passed to
the NetworkStatsFactory includes all underlying interfaces, instead of
only passing the first one.

This (partially) re-applies changes from:
aosp/972848: Add one more test for VPN usage stats.
aosp/972847: Addressing comments for http://ag/7700679.
aosp/885338: NetworkStatsService: Fix getDetailedUidStats to take VPNs
             into account.
Co-developed with: Varun Anand <vaanand@google.com>

Bug: 113122541
Bug: 120145746
Bug: 129264869
Bug: 134244752
Test: FrameworksNetTest passing
Test: Manual tests show data usage fixes maintained.
Change-Id: I6466ec1411fc5ed6954125d27d353b6cd1be719e