Addressing API council comments on NetworkStatsManager.

Callbacks
 - DataUsageCallback renamed to UsageCallback
 - DataUsagePolicy removed; passing in params directly to register method
 - making it an abstract class
 - passing in (networkType, subscriberId) that reached its threshold
 - renaming onLimitReached to onThresholdReached to match existing naming
 - only monitor single network,subscriberId
 - no monitoring of specific uids; using device or user wide instead

Tags
 - only owner uid can read its tags
 - exposing only TAG_NONE to match service side

BUG: 27530098

Change-Id: I2b2664da71806868a1e937d2bf4d1f234637509b