Fix checkapi for OpenJDK 9 builds

This change explitly hides com.android.okhttp
from doclava. The public.txt generation was
previously relying on a package.html in
com.android to hide all subpackages, which
doesn't work with javadoc 9.

Test: make droid (with both OpenJDK 8 and OpenJDK 9)
Bug: 69709687
Change-Id: If61b69f913be044adb4cd34867459bde6330ea2c