Improve window decoration a11y

- Marks unnecessary views as not important for accessibility so that
  they're not announced with Talkback or focused with other a11y
  services (e.g. the app chip should be a single button, not two; the
  icon in the Handle Menu doesn't need to be focusable)
- Removes duplicate Maximize Menu focusable views
- Focuses a11y service on the first view of the Handle and Maximize menu
  after opening them
- Focuses a11y service back on the app chip or maximize menu when their
  respective menu is closed
- Adds missing labels to Maximize Menu's buttons

Fix: 329373346
Fix: 329370431
Fix: 329370508
Fix: 329364244
Fix: 329210454
Fix: 329208268
Fix: 329199667
Fix: 329195839
Flag: EXEMPT bugfix
Test: enable a11y, test scenarios in description
Change-Id: I17540a988b469c912512735da47ccabf4cef1df5