Make TrafficStats uid attribution @SystemApi

Anything that runs as a singleton may need to attribute traffic to
various client apps; in particular, backup transports need to do this.
Apropos of which, introduce a @SystemApi method specifically for that
purpose, setThreadStatsTagBackup().

Bug 16661321

Change-Id: Id5d22e28bdc68edb53f2a1fdba80b144fcbc61d2