Add getMethodHandle to SecurityActions.

Add getMethodHandle to SecurityActions.

The main advantage is that we can do the privileged setAccessible during creation and
then freely invoke via the authorized method handle.

As per the javadocs:
Access checks are applied in the factory methods of Lookup, when a method handle is created.
his is a key difference from the Core Reflection API, since java.lang.reflect.Method.invoke
performs access checking against every caller, on every call.

The performance boost is just a bonus.