AudioService: query a11y manager at init for a11y volume active

When initializing accessibility state monitoring, query the
  accessibility manager whether any service is enabling the
  accessibility volume

Test: adb shell dumpsys audio, look for sIndependentA11yVolume
Bug: 30448020
Change-Id: Ie8c32e10a35f4eef5fdeadacccefaaec1cdfb022