Enable libassistant logging on device

Recently we noticed the libassistant.log file is always empty when
running the Assistant on real hardware.

The root cause for this is that LibAssistant started disabling logging
by default.  This CL enables the flag to enable logging in LibAssistant.

Tests: Deployed on real hardware and verified the logs are populated.
Bug: b/148693233
Change-Id: I2e12742f57649b108e49a825c3d83e8dae4645da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088217Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Cr-Commit-Position: refs/heads/master@{#747821}

chromeos/services/assistant/utils.cc

@@ -109,6 +109,10 @@ std::string CreateLibAssistantConfig(
     // Print logs to console if running in desktop mode.
     internal.SetKey("disable_log_files", Value(true));
   }
+
+  // Enable logging.
+  internal.SetBoolKey("enable_logging", true);
+
   config.SetKey("internal", std::move(internal));
 
   Value audio_input(Type::DICTIONARY);