Process AMS events in NetworkPolicy handler.

When a process changes foreground status or dies, NetworkPolicy
updates its internal state with a lock held.  In cases where there
is contention, this can block the AMS handler and prevent other
events, such as broadcasts, from being dispatched.

This change moves the incoming AMS events to an existing internal
NetworkPolicy handler thread, where they can execute without
blocking AMS.

Bug: 5497544
Change-Id: Ie0c620a620fd9f0f4eb02af510bd819efa4deb6a