Disable notification channel warnings by default

 * Since we usually ship "userdebug" builds, it is better to globally
   disable the toast because we are not in control of third-party apps
   that haven't been properly aligned with new Oreo requirements.

 * Leave it enabled by default only for "eng" builds.

Change-Id: Ie5cdec1cbdd3285af3af730dac4a5de95ad8b334