Change the default parameter values in common.MakeTempFile().

tempfile.TemporaryFile() complains when 'None' is passed as the
prefix/suffix. It uses prefix='tmp' and suffix='' as the default values
and we should do the same.

Test: Call check_ota_package_signature.py and ota_from_target_files.py
and they still work.

Change-Id: I7fb023a3fd0b1a57c009631d0c57a7bb8e4cb5a3
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 3f3b011..4f24ad5 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -990,7 +990,7 @@
   return args
 
 
-def MakeTempFile(prefix=None, suffix=None):
+def MakeTempFile(prefix='tmp', suffix=''):
   """Make a temp file and add it to the list of things to be deleted
   when Cleanup() is called.  Return the filename."""
   fd, fn = tempfile.mkstemp(prefix=prefix, suffix=suffix)