When save person attribute type, only update search index if "searchable" property is saved

When saving a Person Attribute Type, the PersonAttributeType search index is updated (because the searchable property of that attribute may have changed):

https://github.com/openmrs/openmrs-core/blob/master/api/src/main/java/org/openmrs/api/impl/PersonServiceImpl.java#L187

For databases with large numbers of persons/patients, rebuilding the search could take 20-30 minutes (or more).

So, we should explicitly test if the searchable property has changed, and only update the index if this is the case.

(Ideally, the index update could be asynchronous as well, but we can handle that in a separate ticket)