Allow users to set custom Doze settings.

Partially undo I7fff451b641eee43a13e77df7b40db5b3af16d58.
Allow users to customize Doze parameters as they used to be able to do
before.
Also pull out related logic into a tested helper class.

Bug: 305056706
Test: atest CtsBatterySavingTestCases
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/tare
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/tare
Test: atest FrameworksMockingServicesTests:DeviceIdleControllerTest
Test: atest FrameworksServicesTests:BatterySaverPolicyTest
Test: atest FrameworksServicesTests:UserSettingDeviceConfigMediatorTest
Test: atest SettingsProviderTest:SettingsBackupTest
Change-Id: Ieda9ee354b1dde01fed186f19e8ed957b2bbfe00