Creation ios bg session under dedicated ifdef

[rustam-gamidov-here]

There are concerns that bg session creation can take time. Related code is wrapped with OLP_SDK_NETWORK_IOS_BACKGROUND_DOWNLOAD but not the session creation.

Relates-To: MINOR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.29%. Comparing base (ac74097) to head (af4d7ee).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1712      +/-   ##
==========================================
- Coverage   80.33%   80.29%   -0.04%     
==========================================
  Files         356      356              
  Lines       14285    14284       -1     
  Branches     1564     1564              
==========================================
- Hits        11475    11468       -7     
- Misses       2181     2186       +5     
- Partials      629      630       +1     

see 3 files with indirect coverage changes

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