assistant: disable logging to stderr

Bug: b:115397214
Test: locally build and test
Change-Id: Ia7ccdd15906bb195e766f5a93db9e0ed2c0039ca
Reviewed-on: https://chromium-review.googlesource.com/1220812Reviewed-by: Muyuan Li <muyuanli@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592929}

chromeos/services/assistant/utils.cc

@@ -53,6 +53,8 @@ std::string CreateLibAssistantConfig(bool disable_hotword) {
     logging.SetKey(
         "directory",
         Value(GetRootPath().Append(FILE_PATH_LITERAL("log")).value()));
+    // Empty "output_type" disables logging to stderr.
+    logging.SetKey("output_type", Value(Type::LIST));
     config.SetKey("logging", std::move(logging));
   } else {
     // Print logs to console if running in desktop mode.