Rename DND dialogs

A few reasons:
- clarifying which dialog is for the default duration, and which is
  shown every time the user enables DND
- the classes don't directly inherit from Dialog so their names
  shouldn't imply that they do
- "ZenMode" now means more than just DND, so we should make it clear
  the dialogs are for DND only

Flag: EXEMPT trivial rename
Test: builds
Bug: 388261320
Change-Id: I018ac107a3dca8b8776053d7500632eead2ebba1