Javadoc and AudioManager update for A11Y stream

AudioManager:
  - add missing references to STREAM_ACCESSIBILITY
  - add STREAM_ACCESSIBILITY in getDevicesForStream() method
  (this method is only ever called for STREAM_MUSIC)

Test: make offline-sdk-docs
Bug: 30448020
Change-Id: I449082ff01e9822e7e7dca69762c197468237d67