RFR: 8224767: Add String constants for Canonical XML 1.1 URIs

[Sean Mullan]

Please review this fix to add two new String constants to the XML 
Signature API for the Canonical XML 1.1 Algorithm URIs:

webrev: http://cr.openjdk.java.net/~mullan/webrevs/8224767/webrev.00/
CSR: https://bugs.openjdk.java.net/browse/JDK-8224773
Thanks,
Sean