Updates based on API council feedback.

-- Promote generally-useful PFD API to public.
-- Add @WorkerThread annotations.
-- Add docs to explain how AUTHORITY_LEGACY works, and add explicit
START/FINISH_LEGACY_MIGRATION_CALL verbs to communicate progress.
-- Expand docs for createWriteRequest() to explain broader access
that it grants.  Expand all request methods to indicate the action
is fully completed before the result is delivered.
-- Expand docs for new generation fields to explain that version
also needs to be inspected.
-- Explicitly call out why getKindSize() is deprecated, and guide
towards better alternative.

Bug: 148867182, 148403418, 148011561, 147760276
Test: none
Change-Id: I54ff072a752497b6aba153bd4013567c269e5b19