RFR: JDK-8046044 : Fix raw and unchecked lint warnings in XML Signature Impl

[Sean Mullan]

This is a subtask of the task to eliminate all raw and unchecked lint 
warnings from the JDK.

webrev: http://cr.openjdk.java.net/~mullan/webrevs/8046044/webrev.00/
Note that some warnings still require the Suppressed annotation (see 
DOMXMLSignatureFactory, DOMKeyInfoFactory). These can eventually be 
removed when the javax.xml.crypto API is generified, which we will try 
to get to later in the JDK 9 timeframe.

Thanks,
Sean