First pass at NetworkPolicy and activity tracking.

New system service that maintains low-level network policy rules and
collects statistics to drive those rules.  Will eventually connect to
netfilter kernel module through NetworkManagementService and "netd".

Begin tracking foreground activities in ActivityManagerService, which
is updated as part of OOM adjustment.  Eventually a network policy of
POLICY_REJECT_BACKGROUND will reject network traffic from background
processes.

Change-Id: I5ffbbaee1b9628e9c3eff6b9cb2145fc5316e64d