Add a JavaVM.CreateValue() method.

[jonpryor]

Context: Xamarin.Android Bug 25443

The problem is that it is often desirable to dispose of handles ASAP because you "know" that the instances won't be used anymore:

using (var typeface = Typeface.Create (...)) {
    // use typeface
}

The problem is that the above code is making an assumption that Typeface.Create() will be returning a new, unique, instance. If it doesn't (as is actually the case), then the returned wrapper could be shared with other threads, and if the handle is Dispose()d while another thread is using it...Bad Things™ happen.

Addressing this involves two parts; see Issue #4 for the second half.

The first half involves adding another "primitive" operation: in addition to JavaVM.GetValue() (Issue #2), we should have JavaVM.CreateValue() methods which always create a new, possibly aliasing, wrapper instance (if a wrapper would be used, e.g. vm.CreateValue<int>(...) clearly doesn't involve a wrapper...).

We would thus have two ways to turn a JNI handle into a value:

• JavaVM.GetValue() to maintain object identity semantics, maintaining a single wrapper instance per encountered Java instance.
• JavaVM.CreateValue(), which always creates a new wrapper (when required), allowing the caller to always Dispose() of it.