Adding support for adaptive battery saver.

Adaptive battery saver is a state that can be entered into dynamically
without the user turning on full EBS. With this, some features of
battery saver can be enabled to save power before the user needs to have
EBS turned on.

Bug: 119261320
Bug: 32423528
Test: atest android.provider.SettingsBackupTest
Test: atest com.android.server.power.PowerManagerServiceTest
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 CtsBatterySavingTestCases
Change-Id: Ib11ea069828275d788e20cd2e858375eaaea888e