RFR 8205476: KeyAgreement#generateSecret is not reset for ECDH based algorithm

[Adam Petcher]

Webrev: http://cr.openjdk.java.net/~apetcher/8205476/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8205476
CSR: https://bugs.openjdk.java.net/browse/JDK-8212051

Please review the following change for a conformance bug in the ECDH 
service. The KeyAgreement is supposed to reset itself after the call to 
generateSecret, but it is not doing that. I'm also clarifying the spec, 
and this change has the new wording. The CSR was just submitted, and it 
will also need to be approved before this code change is pushed.