Move currentTimeMillis() method to DeviceState

This moves the currentTimeMillis() method from TimeServiceHelper into
DeviceState: currentTimeMillis() is a method that will be used by both
NitzStateMachineImpl and the upcoming NewNitzStateMachineImpl and moving
it to NitzStateMachine.DeviceState means less duplication as it is
shared between both impls. TimeServiceHelper will not be used by
NewNitzStateMachineImpl.

Bug: 140712361
Test: treehugger
Change-Id: I375622be44bd8656b47b3602237ae6718fe8de92