Keep debug information in NetworkStats files

Create two special SETs.

SET_DBG_VPN_IN is used by individual applications to know
how much traffic of the NetworkIdentity was actually moved
from a VPN app.
SET_DBG_VPN_OUT is used by the VPN app to know how much
traffic of the NetworkIdentity was deducted.

A debug application can restore the raw stats by these
entries.
  raw_traffic = recorded_entry (TAG_NONE, SET_ALL)
              + recorded_entry (TAG_NONE, SET_DBF_VPN_OUT)
              - recorded_entry (TAG_NONE, SET_DBF_VPN_IN)

The two debug SETs are not returned by
NetworkStatsService.openSession(). These debug entries are
retrieved by NetworkStatsCollection.dump().

Bug: 19536273
Change-Id: I03ef9f7667f5f2f48cbe3f6b11447fe7ead8ad3b