Collect historical network stats.

Periodically records delta network traffic into historical buckets to
support other services, such NetworkPolicyManager and Settings UI.

Introduces NetworkStatsHistory structure which contains sparse, uniform
buckets of data usage defined by timestamps.  Service periodically
polls NetworkStats and records changes into buckets.  It only persists
to disk when substantial changes have occured.  Current parameters
create 4 buckets each day, and persist for 90 days, resulting in about
8kB of data per network.

Only records stats for "well known" network interfaces that have been
claimed by Telephony or Wi-Fi subsystems.  Historical stats are also
keyed off identity (such as IMSI) to support SIM swapping.

Change-Id: Ia27d1289556a2bf9545fbc4f3b789425a01be53a