google-assistant-sdk/audio_helpers: fix generator termination

Per https://www.python.org/dev/peps/pep-0479/:
"""
the proper way is return, not raise StopIteration.
"""

Bug: 111928212
Change-Id: Iab065bdbe7800f36feeaeba8deb22ab8cbdac525

Author: proppy

google-assistant-sdk/googlesamples/assistant/grpc/audio_helpers.py

@@ -335,7 +335,7 @@ def __iter__(self):
         """Returns a generator reading data from the stream."""
         while True:
             if self._stop_recording.is_set():
-                raise StopIteration
+                return
             yield self.read(self._iter_size)
 
     @property