Add network access blocking when in battery save mode.

Add network access blocking when in battery save mode.

The network policy manager now monitors battery save mode and,
when in battery save, uses its facility to block access to metered
networks to block access to all networks.  That is, it tells the
network management service that all networks have an (infinite)
quota, and puts various app uids to be restricted under quota
interfaces as appropriate.

This new network blocking needs a new facility to be able to white
list apps, such as GmsCore.  To do this, I refactored the package
manager's permission configuration stuff into a separate SystemConfig
class that can be used by others, and it now has a new tag to
specify package names that should be white-listed for power save
mode.  These are retrieved by the network policy manager and used
to build a whitelist of uids.

The new general config files can now go in system/etc/config,
though currently everything still remains in the permissions dir.

Still left to be done is changing the semantics of what uids are
allowed in this mode, to include all perceptable uids.  (So that we
can still do things like background music playback.)  This will be
done in a follow-on CL.

Change-Id: I9bb7029f61dae62e6236da5ca60765439f8d76d2