[iOS] Fixed date formatter issue with different timezone

[jsuarezruiz]

Description of Change

Fixed date formatter issue with different timezone on iOS. From this changes: 938700b#diff-e232660a1722e87a9febc4d293b145e11c6b707d432ae3f637d208e92d70a27d
We use a NSDateFormatter. Since we didn’t specify otherwise, the date formatter uses the system time zone, which is set to the location.
The fix is specify a time zone with a fixed temporal offset instead of a location.

Issues Resolved

• fixes Xamarin.Forms DatePicker shows incorrect date #13366

API Changes

None

Platforms Affected

• iOS

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

1. Set the system datetime to 01/07/21 19:01h and timezone GMT+5.
2. Launch Core Gallery.
3. Look at the DatePicker, the date must be 01/07/21.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)