Prepare release tools for python 3

- Sort dictionaries before looping over them
- Don't call sorted() on lists with Nones
- Open file in binary format when serializing protobufs

Change-Id: If5dbc908f7125f6184014b3c1c7891f833d1d8bf
Bug: 203436762
Test: Presubmits

tools/releasetools/ota_utils.py

@@ -154,7 +154,7 @@ def WriteMetadata(metadata_proto, output):
                 compress_type=zipfile.ZIP_STORED)
     return
 
-  with open('{}.pb'.format(output), 'w') as f:
+  with open('{}.pb'.format(output), 'wb') as f:
     f.write(metadata_proto.SerializeToString())
   with open(output, 'w') as f:
     f.write(legacy_metadata)