Compute range-based usage in NetworkStatsHistory.

When given a start/end range, interpolate between buckets to return
the total network usage.  Used to summarize detailed UID stats.  Method
to combine NetworkStatsHistory regardless of bucket size.  Used to
combine all histories matching a template.

Added tests for both methods.

Change-Id: Ia463910c0ecf7cf08dcf97c658ad99742bd6b882