Removed screen on/off callbacks from NPMS.

NetworkPolicyManagerService (NPMS) used to depend on screen on/off
changes to determine if a foreground activity should have network
restrictions, but such check is now redundant since ActivityManager
already changes the proper UID state (like going from TOP to
TOP_SLEEPING) when the screen status is changed.

Removing such code decreases the NPMS lock contention when the screen is
turned on in about 3-5ms.

Change-Id: I2853443efedbf14961ae9a5b2e72689d4d1a646c
BUG: 30785671