AudioService: fix updating of stream alias for A11y

VolumeStreamState.setAllIndexes():
  - don't do anything if the target and source VSS have the same
    stream type,
  - javadoc of (old+new) behavior.
When STREAM_ACCESSIBILITY becomes independent of any other stream,
  read the settings to restore the volumes previously set.

Test: see bug
Bug: 36286803
Change-Id: I2afcff3a93cc81f3958af252986ef1ac9c38899e