NetworkStatsService to adjust VPN stats before recording.

* Creates a new Parcelable class VpnInfo to hold required
  parameters for VPN stats adjustments.
* ConnectivityService to collect infomation and provide
  a list of VpnInfo, one for each user.
* NetworkStatsService passes the VpnInfo array to
  NetworkStatsRecorder.
* NetworkStatsRecorder calls NetworkStats.migrateTun()
  to do the math.
* Poll NetworkStats when the vpn application calls
  setUnderlyingNetworks().

Bug: 19536273
Change-Id: I7a4c7726b8243fead10416f7ec6eb5cf95f20183