[Time] Truncate TimeDuration length attribute to integer

TimeDuration length attribute in IDL is long long type, it means
that we can accept only integer lengths.

We now use Math.floor() on the length attribute to truncate it.

This patch fixes the TZDate_addDuration_invalidLength TCT test.

Example:

   foo = new tizen.TimeDuration(4.5, "HOURS");

   4.5 -> 4