Tests for NetworkPolicyManager rule generation.

Verifies that policy changes trigger rule updates that respect current
foregroundActivities status.  Also verifies logic that promotes a UID
based on its most-foreground PID.  Verifies that policy changes result
in immediate rule changes.

Also verifies that BACKGROUND_DATA_SETTING_CHANGED broadcasts are sent
by policy changes.

Change-Id: I4fd0dad9e1dbccee2c5968244bb1814e6cb2c6e1