Expose quota status for active network.

Create API to expose quota status derived from underlying network
policy.  This is designed to support applications making informed
decisions when performing network requests.

Fix bug with random stats generation, and write policy when changing
restrict background data flag.  Deprecate EXTRA_NETWORK_INFO, since
it varies based on UID.

Bug: 4517283, 5088603
Change-Id: Ic6893a8967f69937e466be226ba7bb86ef5a5d2d