a11y edits to dnd dialog and volume ring toggle

- don't use announce in the dnd enable dialog - instead make the radio
button content a live region
- add hints to volume dialog ring button and edit content descriptions

Change-Id: I4b0db5e9d8dc6df3f48a4eccad1045d7baca4031
Fixes: 78914992
Fixes: 76385188
Test: manual