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: If0c74bcd03de2b664794e5855c2aaceb5c633b6c
(cherry picked from commit bdccdc5a)