AudioService: remove compile time options for a11y volume

Remove test code path for the a11y volume and only rely on
  AccessibilityServicesStateChangeListener. Query whether
  any of the running a11y services expose
  FLAG_ENABLE_ACCESSIBILITY_VOLUME

Test: enable a service that exposes FLAG_ENABLE_ACCESSIBILITY_VOLUME
  (Talkback will do this in the future) and change volume
Bug 30448020

Change-Id: Ib5dff31313c0ec65b3e9982e35c00698adf5b7de