Create empty PowerWhitelistManager.

This is the first step in extracting the power whitelist from
DeviceIdleController. The whitelist will stay inside the JobScheduler
mainline module boundary, so all calls will have to be made through a
stable API. Eventually, all users of IDeviceIdleController will be
migrated to this interface.

Bug: 140141678
Bug: 141155196
Bug: 144864180
Test: atest PowerWhitelistTest
Change-Id: I8468e74ac8fe6611bfa957d4f5093c8d15be1299