RFR JDK-8215300: additional changes to constants API

[Vicente Romero]

Hi,

Please review some final changes to the constants API. The changes 
should make the API clearer and more precise and were recommended in the 
CSR [3]

Thanks,
Vicente

[1] (jira issue) https://bugs.openjdk.java.net/browse/JDK-8215300
[2] (webrev) http://cr.openjdk.java.net/~vromero/8215300/webrev.00/
[3] https://bugs.openjdk.java.net/browse/JDK-8202031