ensure that app.Categories is always a list of strings

This just makes it easier for people writing build recipes.  Rewriting will
output a list of strings as well.

The test index.xml and categories.txt are updated to include the new number
categories, and the changed CurrentVersionCode to 2147483647 (MAX_VALUE)

fdroidserver/metadata.py

@@ -813,6 +813,11 @@ def post_metadata_parse(app):
         if type(v) in (float, int):
             app[k] = str(v)
 
+    if isinstance(app.Categories, str):
+        app.Categories = [app.Categories]
+    else:
+        app.Categories = [str(i) for i in app.Categories]
+
     builds = []
     if 'builds' in app:
         for build in app['builds']: