[Time] Add getTimezoneOffset, getTimeWithOffset and fix get/set UTCs

getTimezoneOffset replaced getTimezoneRawOffset, this new one calculates
the DST offset.

Fix getters and setters of UTCs using getTimeWithOffset.
[Time] Change getDSTTransition return from undefined to null

Based on the tests:

TZDate_getNextDSTTransition.html
TZDate_getPreviousDSTTransition.html
[Time] Properly handle getTimeFormat