[roll] Roll fuchsia [sestarnix] Index policy classes by-Id and remove _custom() APIs

The SEStarnix `ObjectClass` is now a small, copyable Id container,
holding either a `KernelClass` enum value, or a policy-defined
`ClassId`. This allows callers to lookup & cache `ClassId` values
for later use to perform policy queries efficiently.

All name-based (aka `*_custom()`) query APIs, and the duplicate
resolution logic they contained, are removed, with the
`PolicyIndex` doing the work of resolving either `ObjectClass` form
to locate the policy class, or to apply the handle-unknown
treatment if not found.

The `ObjectClass` ergonomics are slightly improved, to allow more
seamless creation of instances from `KernelClass` values, and the
top-level `Policy` APIs updated to accept any class value capable
of being converted into an `ObjectClass.

Original-Bug: 411025073
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1254084
Original-Revision: 9b66fc860b649e26195134d87c4bed4513642af7
GitOrigin-RevId: 67c70fe9380fe36a35c3dde13d7fba57b44c0aaf
Change-Id: Ief91fb95a092cc5f664d24f75b3eee5e7c15c365