Improve a11y announcements for snoozing.

I also updated the content description for the "Undo" button so it's
clear it's part of the notification snooze functionality. I left the
action as simply "Undo" (alongside "15 minutes", "30 minutes" etc), as
there it's already clear what the context is.

Fix: 307712599
Test: manually enabling snoozing with talkback on
Flag: NONE
Change-Id: I5c22af098b7506b5f4a4870e0eb72cdfc804263e