[generator] Always use XAPeerMembers for XAJavaInterop1.

[jpobst]

Broken out of #341.

When generating the _members field for XAJavaInterop1, in one instance we generate:

... static JniPeerMembers _members = new XAPeerMembers ("android/text/foo", typeof (ISpannableInvoker));

while in the 3 other instances we generate:

... static JniPeerMembers _members = new JniPeerMembers ("android/text/foo", typeof (ISpannableInvoker));

This refactors all the instances of writing this line to a single method which correctly uses XAPeerMembers when needed.