Fixed: Download notification was disappearing when the application is in background.

[MohitMaliFtechiz]

Fixes #4106

• Introduced a foreground service(with dataSync type) for downloading, now it will show the notification of downloading ZIM files even if the application is killed. Since we are using the DownloadManager to download ZIM files, it continues downloading files even when the app is killed. So this foreground service will show a notification with the progress of downloading ZIM files.
• Since we are now using foreground service for downloading functionality for both app and custom apps we have to give an explanation for this to playStore while uploading the custom apps as we have done for Kiwix app(hotspot functionality).
• Fixed the CustomDownloadFragment was not showing if there is no ZIM file available in custom apps.

[codecov]

Codecov Report

Attention: Patch coverage is 82.82051% with 67 lines in your changes missing coverage. Please review.

Project coverage is 58.81%. Comparing base (fff3c3e) to head (05891eb).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...wnloader/downloadManager/DownloadMonitorService.kt	80.25%	48 Missing and 15 partials ⚠️
...wnloader/downloadManager/DownloadManagerMonitor.kt	85.00%	2 Missing and 1 partial ⚠️
...der/downloadManager/DownloadNotificationManager.kt	98.03%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4108      +/-   ##
============================================
+ Coverage     58.49%   58.81%   +0.31%     
- Complexity     1535     1558      +23     
============================================
  Files           316      316              
  Lines         13351    13421      +70     
  Branches       1700     1707       +7     
============================================
+ Hits           7810     7893      +83     
+ Misses         4408     4392      -16     
- Partials       1133     1136       +3     

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