[Alarm] Initialize the "id" to null in constructor
The alarm id becomes valid only when it's added into tizen system.
Before that we should ensure the property is enumerable.
Set the configurable to true because we can re-define the property later in sub class when it's added into tizen system.

BUG=XWALK-2127
Merge pull request #354 from yejingfu/topic-alarm

[Alarm] Initialize the "id" to null in constructor