Data usage warning and limit notifications.

Watch for network statistics to cross NetworkPolicy warning or limit,
and show notifications to user as needed.  Currently checks during
any statistics update, but will eventually move to event registration
through netd when kernel supports.

Fixed accounting bug in getSummaryForNetwork().  Only apply UID policy
to applications; applying to system processes could break critical
services like RIL.

Change-Id: Iac0f20e910e205f3cbc54ec96395ff268b1aa379