Proactively disable data when over policy limit.

Add policy controls to NetworkStateTracker which are combined with
other user preference and internal flags to decide if data connection
should be established.  Better locking around enabled flags.

When data network would be over limit, proactively disable data on
that network.  Enable when policy is snoozed or when cycle resets.

Track and dismiss notifications from now-stale policies.

Bug: 4587023, 5178147
Change-Id: Ibfcc9f73cda7c369209af701b46eddd3d1943f2d