[Time] Fix TimeDuration getters and setters variable
[Time] Fix TimeDuration length

TimeDuration length is long long type (IDL), we now add a Math.floor() on the
lenght to truncate it.

This patch fixes the TZDate_addDuration_invalidLength tct test.

Ex:

foo = new tizen.TimeDuration(4.5, "HOURS");

4.5 -> 4