copy icons out of the APKs into the FDroid repo icons folder

This gets the apps icons from the APK and makes it part of the local FDroid
repo, so that when people view the apps in this repo, they will see the
nice icon :)

closes #2694
run icons copy in background AsyncTask, its optional

The user does not need to wait for the icons copy to finish in order to use
the new repo, it will work fine without the icons.  So put the icons copy
in an AsyncTask without a progress dialog so that it happens in the
background while the user can do other things.

refs #2694