Add DnD as a special case Slice

Add DND Slice as a special case, since there is an existing
inheritance structures in the zen mode preference controllers which
would be too risky to change at this point in the release.

Change-Id: If4b7013be35c89695786af2dbbea2edcf7a189f3
Merged-In: Ice608b9a7bd6f38b73e581eb3723f0a2fae96f2b
Test: make RunSettingsRoboTests
Fixes: 67997377