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