Resolve 3 inconsistencies in accessibility button API

- Unify logic for detecting availability of the accessibility button
- Ensure the initial visibility state is propagated to A11yMS
- Ensure services only receive availability callbacks for changes

Test: Manual, created test accessibility services
      targeting specific breakages
Bug: 38345417

Change-Id: I2250b32830cdfc2ecdc1dff7b7130dced2c1db29