Improve javadocs for a11y live regions and minimum durations

For live regions:
- Make descriptions generally  more concise.
- Reformat the docs so there is more structure (i.e. lists)
- Separate behavior from semantics by separating notifications from
  announcements
- Discourage live regions for UI that frequently updates
- Link to setMinDurationBetweenContentChanges

For min durations:
- Add examples of widgets that could use durations
- Link to Progressbar

Bug: 332393061
Test: n/a (docs)
Change-Id: I6fcdda3d15d45a572cc9def39e8b3a640a39687c