Add provideAssistData attr to public.xml

http://ag/323631 added the provideAssistData
attribute to the Service manifest tag.

It did not, however, add that attribute to
public.xml - making it impossible to actually
build a service that defines said tag.

So, add it.

Now that the attribute is where it should be,
restore @link notation to ServiceInfo.java
without breaking the offline docs build
( see http://ag//340279 ).

Also, make some log warnings related to
providing assist data slightly more verbose.

Bug: 10573008
Change-Id: Ie2bcb411c182d69738a2fa4a74de3171b9b9c455