Credits

Call Prefix Filter takes source code from a lot of different places. The internet community deserves much credits. A lot of individuals generously share their knowledge.

Action Bar in settings page

The AppCompatDelegate sample code is found in Stack Overflow.

https://stackoverflow.com/questions/17849193/how-to-add-action-bar-from-support-library-into-preferenceactivity

File picker

The file picker dialog box is based on Roger Keays' article.

https://rogerkeays.com/simple-android-file-chooser

ListPreference summary

The sample code is posted by Uncle Code Monkey.

https://stackoverflow.com/questions/531427/how-do-i-display-the-current-value-of-an-android-preference-in-the-preference-su

Notification in Gingerbread

The Java reflection code is posted by Vincent Hiribarren.

https://stackoverflow.com/questions/32345768/cannot-resolve-method-setlatesteventinfo

USB file presence

Cindy Potvin's article points out the use of MediaScannerConnection.

https://blog.cindypotvin.com/saving-data-to-a-file-in-your-android-application/