Introduce enable_on_device_assistant GN arg

Bug: b/154866551
Change-Id: I6f2de3a8d9e3e7bb99c665bc1eedeae1ff775e15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2197151
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Auto-Submit: Jeroen Dhollander <jeroendh@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#767966}

chromeos/assistant/assistant.gni

@@ -3,6 +3,7 @@ import("//build/config/chrome_build.gni")
 declare_args() {
   # Enable assistant implementation based on libassistant.
   enable_cros_libassistant = is_chromeos && is_chrome_branded
+  enable_on_device_assistant = false
 }
 
 declare_args() {