Foreground/background network stats using sets.

Teach NetworkStats about "counter sets" coming from kernel, and use
them to track usage in foreground/background.  Add AID_NET_BW_ACCT to
system_server so it can control counter sets.

Move to composite key of NetworkIdentitySet, UID, set, and tag when
recording historical usage.  Persisting still clusters by identity,
since that is heaviest object.

Request async stats poll during systemReady() to bootstrap later
delta calculations. Reset kernel counters when UID removed. Update
various tests.

Bug: 5105592, 5146067
Change-Id: Idabec9e3ffcaf212879821515602ecde0a03de8c