The POLICY_REJECT_BACKGROUND policy requires that network traffic be
blocked when a UID goes into the background. Even if the UID has an
activity in the foreground, it's considered "background" if the screen
is turned off.
This changes watches for SCREEN_ON/OFF broadcasts, and rule generation
now observes screen state. It also introduces an observer pattern so
that ActivityManager doesn't directly know about NetworkPolicy, and
moves the service management into SystemServer.
Change-Id: Ie7a84929d3ca60ae4578d47e19d5a8da10fd8d58
a462079303