Add A11y Slices

Add AccessibilityPreferenceController, which wraps all a11y settings
since they are share common infrastructure for enabling, current value,
and availability.

We add an overlay for OEMs to declare their bundled a11y services.
This is the only list of services that will be possible to enabled via
Settings slices.

Accessibility Slices are built by getting a list of valid services,
and indexing the service names as a key in the Slices DB. When they are
built at runtime, they use the generic A11yPrefController to get the status
and enable/disable the service.

Bug: 67997836
Bug: 67997672
Test: robotests
Change-Id: I66f905bf1c55eecb937945c4675c12bcbc96d698