Refactored BS settings into Policy subclass.

Creating a class/object that details the specific policy will make it
easier to have multiple different policies simultaneously that are
well-defined and in one place.

Bug: 119261320
Test: atest com.android.server.power.BatterySaverPolicyTest
Change-Id: I8ba89ff2f308ffcdedc09739e513734d200eee56