Use constants for field/flag types

Avoids typos, such as one I just found which was 'strsng' isntead of
'string'. The static analyzer can catch those if they are constants.
Comparing ints is also faster than strings, which adds up in readmeta.

fdroidserver/lint.py

@@ -108,7 +108,7 @@ def check_regexes(app):
         for m, r in checks:
             v = app.get_field(f)
             t = metadata.metafieldtype(f)
-            if t == 'multiline':
+            if t == metadata.TYPE_MULTILINE:
                 for l in v.splitlines():
                     if m.match(l):
                         yield "%s at line '%s': %s" % (f, l, r)