Rename JavaVM.GetObject() to JavaVM.GetValue().

[jonpryor]

Rename JavaVM.GetObject() to JavaVM.GetValue().

Rationale: I want to remove the IJavaObject constraint so that we can do e.g. vm.GetValue<int>(javaLangIntegerHandle) to read a java.lang.Integer into an int (ditto arrays, etc.), and "Object" no longer makes sense when a copy of the value will be created.