[Alarm] Fix failures raised from test suite.

I've downloaded the test suite from http://otcqa.sh.intel.com/qa-auto/live/Web_TCT_Xwalk/ and run them manually.
All the alarm related failures in the test suite have been fixed.

BUG=XWALK-2163
> Why we don't throw error in this case?

This change can ensure passing the following test case (in AlarmRelative_getRemainingSeconds_return_null.html):

          returnedValue = alarm.getRemainingSeconds();
          assert_equals(returnedValue, null, "getRemainingSeconds do not returns null");

So according to the spec (and the test case), if the alarm has been triggered, the getRemainingSeconds() should return null instead of throwing an exception.