Refactored TimeZoneInfo.GetUtcOffset.
Added GetUtcOffset auxiliar methods with bool out parameter that is true when the given DateTime is a DST.
Use TZ transitions when AdjustementRules are ignored. Fixes 23170.
In some cases the timezone transitions provided by iOS are not convertible to AdjustmentRules because of changes on the base offset.
Now we store the transitions so they can be used instead of an incomplete list of AdjustmentRules.