Add metadata hash to LocalTransport logs

e.g https://paste.googleplex.com/5891511475503104

Bug:268471749
Test: 1. Turn on logging of the events to logcat
         (adb shell settings put secure
	 backup_local_transport_parameters "log_agent_results=true")
      2. Run backup for wallpaper
         (adb shell bmgr backupnow com.android.wallpaperbackup)
      3. Verify metadata hash is included in logcat
         (adb logcat -s LocalTransport)
Change-Id: I256e9cead21f01ea9f68a18e5262326d4b4a1f07