Persist "tagged" network stats along with UIDs.

Now stores tags other than TAG_NONE (0x0), which are useful for app
debugging.  Combine UID and tag together into single long key, and
expose tag data through AIDL when requested.  Change NMS to track
TAG_NONE as total UID traffic, matching the kernel definition.

Added TAG_MAX_HISTORY to control how long tag-granularity statistics
are stored; overall UID usage is still kept for UID_MAX_HISTORY.  Fix
bug to trim NetworkStatsHistory outside normal polling loops to catch
non-active networks and UIDs.

Test to verify UID and tag packing, and to verify that UID traffic on
two networks are combined to match MOBILE_ALL template.

Change-Id: If0e039416d9e7f63b1a39e04cddfb1133b5a78ee