Implementation of data usage callbacks.

NetworkStatsService will register data usage requests
and keep data usage stats scoped to the request.

There are different types of data usage requests
- scoped to a set of NetworkTemplate; these are restrictred to
device owners and carrier apps and allow the caller to monitor
all activity on the specified interfaces.
- scoped to all uids visible to the user, if the user has
android.Manifest.permission#PACKAGE_USAGE_STATS permission.
The set of uids may change over time, so we keep track of that.
- scoped to a set of uids given by the caller, granted that
the caller has access to those uids.
- scoped to the caller's own data usage. This doesn't require
PACKAGE_USAGE_STATS.

Bug: 25812785
Change-Id: Ie11f35fc1f29d0dbe82f7fc924b169bb55c76708