Store preferences to its default path

As documented  in the Defaults Files section of the extension documentation [0], “Default preferences .js files should be stored in defaults/preferences/ - when you place them here they will be automatically loaded by Firefox's preferences system when it starts so that you can access them using the Preferences API.”

  0: https://developer.mozilla.org/en-US/docs/Building_an_Extension#Defaults_Files

Bug-Debian: https://bugs.debian.org/684390
Bug: https://adblockplus.org/forum/viewtopic.php?f=11&t=16529