Migrate remote configuration to DeviceConfig.

Move battery saver to retrieve remote configuration values via
DeviceConfig. Some products allow users to customize which battery saver
toggles are active, so leave the Global settings flag around to track
the user's choices. The user's choices will override anything sent from
the server.

Also renaming some of the flag names since we don't need to support
backwards compatibility for server-side pushes.

Bug: 79580230
Bug: 124466289
Test: atest com.android.server.power.batterysaver.BatterySaverPolicyTest
Test: atest com.android.server.power.batterysaver.BatterySaverStateMachineTest
Test: atest com.android.server.power.batterysaver.BatterySavingStatsTest
Test: atest com.android.server.power.PowerManagerServiceTest
Test: atest CtsBatterySavingTestCases
Test: atest CtsLocationFineTestCases
Change-Id: I47684efa5c4426e1c524fb9c1d4d407ee66fa3b7