Introduce legacy settings restore whitelists

On rare occasion we have settings keys that we no longer want to
back up, but which we *do* want to process at restore time for
support of legacy backup datasets.  We now have a little bit of
machinery for the idea of restore-only key whitelists, paralling
the SETTINGS_TO_BACKUP dual-purpose whitelists.

Bug 62263757
Test: manual

Change-Id: Ic32ad679bc4b5028dd52dc71e9ab5836a76307c4