Rework build into a class

This simplifies usage, goes from

    build['flag']
to
    build.flag

Also makes static analyzers able to detect invalid attributes as the set
is now limited in the class definition.

As a bonus, setting of the default field values is now done in the
constructor, not separately and manually.

While at it, unify "build", "thisbuild", "info", "thisinfo", etc into
just "build".

fdroidserver/import.py

@@ -223,20 +223,15 @@ def main():
         sys.exit(1)
 
     # Create a build line...
-    build = {}
-    build['version'] = version or '?'
-    build['vercode'] = vercode or '?'
-    build['commit'] = '?'
-    build['disable'] = 'Generated by import.py - check/set version fields and commit id'
+    build = metadata.Build()
+    build.version = version or '?'
+    build.vercode = vercode or '?'
+    build.commit = '?'
+    build.disable = 'Generated by import.py - check/set version fields and commit id'
     if options.subdir:
-        build['subdir'] = options.subdir
+        build.subdir = options.subdir
     if os.path.exists(os.path.join(root_dir, 'jni')):
-        build['buildjni'] = ['yes']
-
-    for flag, value in metadata.flag_defaults.iteritems():
-        if flag in build:
-            continue
-        build[flag] = value
+        build.buildjni = ['yes']
 
     app.builds.append(build)