Update flag names.

Since we just migrated DeviceIdleController to DeviceConfig, we can take
the opportunity to clean up and rename the flags for better readability.

Bug: 166121524
Test: atest DeviceIdleTest
Test: atest FrameworksMockingServicesTests:DeviceIdleControllerTest
Test: atest CtsSecurityTestCases:DeviceIdleControllerTest
Change-Id: I322dd08a1d7d4232f4716b264b1d768198e70457