upgrade zxing library to v3.0.1
move getDefaultRepoName() to org.fdroid.fdroid.Utils

org.fdroid.fdroid.Utils is the current FDroid location for static utility
methods.

refs #3204 https://dev.guardianproject.info/issues/3204
clean up Utils.java for easier merging
refactor webserver class into much simplified NanoHTTPD subclass

The SimpleWebServer.java example had a lot more stuff than we needed in it,
plus it had some code that was only supported on android-9 or higher. I
stripped out anything that is not currently needed and merged
KerplappHTTPD and SimpleWebServer into a single class: LocalHTTPD.

This should now work down to android-7 at least.

refs #3204 https://dev.guardianproject.info/issues/3204
when changing "Use HTTPS", use AsyncTask to keep things responsive

The process of configuring and kicking off the Service to handle the
changes needed can mean that the Settings Activity pause. Putting that into
an AsyncTask ameliorates that.
sync up Utils.getSharingUri() implementations
rename kerplapp_activity.xml to local_repo_activity.xml

Matches the Java class name.
rename resources to ease syncing
rename SettingsActivity to PreferencesActivity to sync with FDroid
remove debug code to keep in sync
code formattting
move Comparator to a private class, its only used in AppListLoader
purge unused code from AppListLoader
add a description string to <application> in the manifest