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.

Test: make RunSettingsRoboTests
Bug: 67997377
Change-Id: Ice608b9a7bd6f38b73e581eb3723f0a2fae96f2b