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>