AudioService: modify logic for independent a11y volume

Add support for enabling independent a11y volume based on
  a11y services state change.
Add compile time constant USE_FLAG_ENABLE_ACCESSIBILITY_VOLUME
  for testing a11y volume simply when TalkBack is enabled.

Test: change new USE_FLAG... constant to false and enable Talkback
Bug 30448020
Change-Id: I541481ed6ca24bc46872818bb5306c5662ab80e9