[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
According to the test case `Alarm_id_attribute_relative_alarm.html` (and spec), the `id` attribute should be string instead of number.
The `''+ret.data['id']` converts number to string and it's faster than `ret.data['id'].toString()`.