properly parse build metadata list types like gradle=

Something like `gradle: yes` in YAML will be parsed as a boolean, since
'yes' is officially defined as a boolean true in YAML.  For metadata fields
that need to be lists, this needs to be converted.  Same goes for a single
string like `gradle: customFlavor`.

fdroidserver/metadata.py

@@ -904,6 +904,11 @@ def post_metadata_parse(app):
             elif ftype == TYPE_STRING:
                 if isinstance(v, bool) and v:
                     build.__dict__[k] = 'yes'
+            elif ftype == TYPE_LIST:
+                if isinstance(v, bool) and v:
+                    build.__dict__[k] = ['yes']
+                elif isinstance(v, str):
+                    build.__dict__[k] = [v]
 
     if not app.Description:
         app.Description = 'No description available'