code cleanup

* 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)

app/src/main/res/values-ja/strings.xml

@@ -456,7 +456,6 @@
     <string name="saved_tabs_invalid_json">デフォルトのタブを使用します。保存されたタブの読み込みエラーが発生しました</string>
     <string name="main_page_content_summary">メインページに表示されるタブ</string>
     <string name="updates_setting_description">新しいバージョンが利用可能なときにアプリの更新を確認する通知を表示します</string>
-    <string name="msg_pending_downloads">ダウンロードから %s の保留中の転送を続行します</string>
     <string name="pause_downloads_on_mobile">従量制課金ネットワークの割り込み</string>
     <string name="pause_downloads_on_mobile_desc">モバイルデータ通信に切り替える場合に便利ですが、一部のダウンロードは一時停止できません</string>
     <string name="show_comments_title">コメントを表示</string>