Update custom zen settings "off" string

zen_mode_sound_summary_off should only be used when describing the
number of schedules that are enabled (and requires a numeric parameter).
Instead, use the string "off" when a sound isn't enabled by a custom dnd
setting.

Fixes: 189120026
Test: manual
Change-Id: I8bc180353a61455b124801d14f0d319f49bb341c