Move some static methods from NPMS to NPM class.

Moved isProcStateAllowedWhileIdleOrPowerSaveMode and
isProcStateAllowedWhileOnRestrictBackground to
NetworkPolicyManager from NetworkPolicyManagerService
so that these can be accessed from other classes.

Bug: 27803922
Test: Builds without any errors.
Change-Id: I4736652d546cdc84de3cdea9b6524854e4773419