AudioService: prevent muting of alarm / accessibility

STREAM_ACCESSIBILITY / ALARM min volume is index 1

Prevent a11y stream volume to be 0 when used separately from
  media.
Prevent setting a volume index above min value.

Bug: 72403580
Bug: 74985293
Test: verify media vol can go to 0 ; enable Talkback verify
    a11y volume can't be muted with vol keys or UI

Change-Id: Id46506d4c407f42fe3aca463055b0973e1a7c4dc