Fix DCHECK in audio_input_provider

Bug: b/112704035
Test: Manual Test
Change-Id: I83e3acab1399e3e43e5c9e9f3022c96809a3881e
Reviewed-on: https://chromium-review.googlesource.com/1179301Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584120}

chromeos/services/assistant/platform/audio_input_provider_impl.cc

@@ -137,6 +137,10 @@ void AudioInputImpl::RemoveObserver(
     task_runner_->PostTask(FROM_HERE,
                            base::BindOnce(&AudioInputImpl::StopRecording,
                                           weak_factory_.GetWeakPtr()));
+
+    // Reset the sequence checker since assistant may call from different thread
+    // after restart.
+    DETACH_FROM_SEQUENCE(observer_sequence_checker_);
   }
 }