Adapt Datetime parser to DST timezones

[diachenko-mischa]

When iOS device locale is set to location
which uses different timezeons depending on
daylight saving time (eg PST and PDT for
Los Angeles), then different gmtoff applied
by strptime for 'summer' and 'winter'
timestamps.

This results in the wrong translation of
datetime string into epoch.

Adapt code to iOS/MacOS behaviour, where
timegm updates an input parameter and
erases gmtoff value:

• remove logic to infer the offset using epoch datetime;
• store gmtoff value before calling timegm;
• offset result returned from timegm by saved gmtoff.

Relates-To: HERESDK-2568

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.45%. Comparing base (3dddc67) to head (3e44c6f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1524      +/-   ##
==========================================
- Coverage   81.47%   81.45%   -0.01%     
==========================================
  Files         344      344              
  Lines       13542    13538       -4     
  Branches     1455     1456       +1     
==========================================
- Hits        11032    11027       -5     
- Misses       1917     1919       +2     
+ Partials      593      592       -1     

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