Establish permanently unavailable settings

Distinguish between settings which are permanently unavailable on
the device, and temporarily unavailable. This enables us to restrict
which setting slices are exposed in onSliceGetDescendants.

The primary changes in this CL are renaming:
"DISABLED_UNSUPPORTED" -> "UNSUPPORTED_ON_DEVICE"
to be more clear the the setting will cannot be accessed on the device, and,
adding a new enum to encapsulate settings which are currently unavailable, but
could be enabled in the future.

Also remove UNAVAILABLE_UNKNOWN. Devs should never need this enum.

Bug: 78910582
Fixes: 79245656
Test: robotests
Change-Id: I42c2cedab66be2d76999795f46470a079cc1ec71
Merged-In: I58821a6cfd6134b3b351657b6edf5f74ead00643