clea-lib java : small fixes

Adding some utility functions in TimeUtils for Instant to NTP timestamp conversions. fixing a bug in Location.setQrCodeValidityStartTime() that would throw an ArithmeticException (/ by zero) if the QrRenewalInterval was 0, i.e a static Cléa QR code. fixing Location.newDeepLink(periodStartTime) that would fire a StackOverflowError because of infinite recursion. modifying Location.contact.periodStartTime when Location.setPeriodStartTime() is called.