bind,internal/importers: add Unwrap methods to unwrap Java wrappers

For Java classes implemented in Go, it is useful to take a Java instance
and extract its wrapped Go instance. For example, consider the
java.lang.Runnable implementation wrapping a Go function:

package somepkg

type GoRunnable struct {
    lang.Runnable
    f func()
}

Java methods that take a java.lang.Runnable cannot directly take a
*GoRunnable, so this CL adds a Unwrap method:

import gorun "Java/somepkg/GoRunnable"

...

r := gorun.New()
r.Unwrap().(*GoRunnable).f = func() { ... }
javapkg.Run(r)

The extra interface conversion is unfortunately needed to avoid
import cycles.

Change-Id: Ib775a5712cd25aa75a19d364a55d76b1e11dce77
Reviewed-on: https://go-review.googlesource.com/35295
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/ClassesTest.java

@@ -18,6 +18,7 @@ import javapkg.GoObject;
 import javapkg.GoRunnable;
 import javapkg.GoSubset;
 import javapkg.GoInputStream;
+import javapkg.GoArrayList;
 
 public class ClassesTest extends InstrumentationTestCase {
 	public void testConst() {
@@ -148,4 +149,9 @@ public class ClassesTest extends InstrumentationTestCase {
 		Runnable r4c = Javapkg.castRunnable(new Object());
 		assertTrue("Invalid cast", r4c == null);
 	}
+
+	public void testUnwrap() {
+		GoArrayList l = new GoArrayList();
+		Javapkg.unwrapGoArrayList(l);
+	}
 }