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
6f7af03cf1