Move network stats to FileRotator pattern.

Split existing network stats into two separate classes: a recorder
which generates historical data based on periodic counter snapshots,
and a collection of historical data with persistance logic.

Recorder keeps a pending history in memory until outstanding data
crosses a specific threshold.  Persisting is handled through a given
FileRotator.  This pattern significantly reduces disk churn and
memory overhead.  Separate UID data from UID tag data, enabling a
shorter rotation cycle.  Migrate existing stats into new structure.

Remove "xt" stats until iptables hooks are ready.  Avoid consuming
Entry values when recording into NetworkStatsHistory.  Assign
operation counts to default route interface.

Introduce "Rewriter" interface in FileRotator with methods to enable
rewriteAll().  Introduce IndentingPrintWriter to handle indenting in
dump() methods.

Bug: 5386531
Change-Id: Ibe086230a17999a197206ca62d45f266225fdff1