applets/notification: improve DND mode explanatory text

This text suffered from two issues:
- By trying to generate an English sentence, it became a string puzzle
  because not all possible relative date strings would work after the
  word "until". For example, "Until In two days" would be wrong.
- It used inappropriate capitalization of the date despite trying to be
  in a sentence.

This commit fixes both issues by splitting the two parts of the text on
either side of a colon, creating a logical break between them.

BUG: 421989
FIXED-IN: 5.27