[assistant.picovoice] Conversation flow improvements.

- The `Responding` state should be modelled as an extra event/binary
  flag, not as an assistant state. The assistant may be listening for
  hotwords even while the `tts` plugin is responding, and we don't want
  the two states to interfere with each either - neither to build a more
  complex state machine that also needs to take concurrent states into
  account.

- Stop any responses being rendered upon the `tts` plugin when a new
  hotword audio is detected. If e.g. I say "Ok Google", I should always
  be able to trigger the assistant and stop any concurrent audio
  process.

- `SpeechRecognizedEvent` should be emitted even if `cheetah`'s latest
  audio frame results weren't marked as final, and the speech detection
  window timed out. Cheetah's `is_final` detection seems to be quite
  buggy sometimes, and it may not properly detect the end of utterances,
  especially with non-native accents. The workaround is to flush out
  whatever text is available (if at least some speech was detected) into
  a `SpeechRecognizedEvent` upon timeout.

platypush/plugins/assistant/picovoice/_recorder.py

@@ -42,11 +42,17 @@ class AudioRecorder:
         )
 
     def __enter__(self):
+        """
+        Start the audio stream.
+        """
         self._stop_event.clear()
         self.stream.start()
         return self
 
     def __exit__(self, *_):
+        """
+        Stop the audio stream.
+        """
         self.stop()
 
     def _audio_callback(self, indata, *_):
@@ -59,6 +65,13 @@ class AudioRecorder:
             self.logger.warning('Audio queue is full, dropping audio frame')
 
     def read(self, timeout: Optional[float] = None):
+        """
+        Read an audio frame from the queue.
+
+        :param timeout: Timeout in seconds. If None, the method will block until
+            an audio frame is available.
+        :return: Audio frame or None if the timeout has expired.
+        """
         try:
             return self._audio_queue.get(timeout=timeout)
         except TimeoutError:
@@ -66,6 +79,9 @@ class AudioRecorder:
             return None
 
     def stop(self):
+        """
+        Stop the audio stream.
+        """
         self._stop_event.set()
         self.stream.stop()
 
@@ -73,4 +89,7 @@ class AudioRecorder:
         return self._stop_event.is_set() or self._upstream_stop_event.is_set()
 
     def wait(self, timeout: Optional[float] = None):
+        """
+        Wait until the audio stream is stopped.
+        """
         wait_for_either(self._stop_event, self._upstream_stop_event, timeout=timeout)