* migrate few annotations to androidx
* mission recovery: better error handling (except StreamExtractor.getErrorMessage() method always returns an error)
* post-processing: more detailed progress
[file specific changes]
DownloadMission.java
* remove redundant/boilerplate code (again)
* make few variables volatile
* better file "length" approximation
* use "done" variable to count the amount of bytes downloaded (simplify percent calc in UI code)
Postprocessing.java
* if case of error use "ERROR_POSTPROCESSING" instead of "ERROR_UNKNOWN_EXCEPTION"
* simplify source stream init
DownloadManager.java
* move all "service message sending" code to DownloadMission
* remove not implemented method "notifyUserPendingDownloads()" also his unused strings
DownloadManagerService.java
* use START_STICKY instead of START_NOT_STICKY
* simplify addMissionEventListener()/removeMissionEventListener() methods (always are called from the main thread)
Deleter.java
* better method definition
MissionAdapter.java
* better method definition
* code cleanup
* the UI is now refreshed every 750ms
* simplify download progress calculation
* indicates if the download is actually recovering
* smooth download speed measure
* show estimated remain time
MainFragment.java:
* check if viewPager is null (issued by "Apply changes" feature of Android Studio)
Remove strings which have been deleted from the English strings file during development, but were translated via Weblate, which failed to pull and push our upstream repo.
