Rename command-line flag OnDeviceAssistant

The old name, ChromeOsOnDeviceAssistant, is simply too long and
the |ChromeOs| doesn't add anything extra.

Change-Id: Ie7e742672a7440d22404366ccbe92b35f80da89c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225261Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Cr-Commit-Position: refs/heads/master@{#773766}

chromeos/services/assistant/public/cpp/features.cc

@@ -46,7 +46,7 @@ const base::Feature kEnableStereoAudioInput{"AssistantEnableStereoAudioInput",
 const base::Feature kEnablePowerManager{"ChromeOSAssistantEnablePowerManager",
                                         base::FEATURE_DISABLED_BY_DEFAULT};
 
-const base::Feature kEnableOnDeviceAssistant{"ChromeOSOnDeviceAssistant",
+const base::Feature kEnableOnDeviceAssistant{"OnDeviceAssistant",
                                              base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kEnableMediaSessionIntegration{