Add javadoc for all public ahat API.

And tighten up the API a little more.

The only functional change is to replace the Sort.WithPriority class
with a Sort.withPriority function so that the Sort.WithPriority class
can be made private.

Test: m ahat-test
Change-Id: Iba0ac04767a20c9008f209a777294ecaccc64ffc