example/libhello: update main.go to initiate java.Init.

changes required after golang.org/cl/5555

Change-Id: If355dcfa0edfdc204eac4e89837927ab9a978b65
Reviewed-on: https://go-review.googlesource.com/5651
Reviewed-by: David Crawshaw <crawshaw@golang.org>

example/libhello/main.go

@@ -11,10 +11,10 @@ package main
 import (
 	"golang.org/x/mobile/app"
 
-	_ "golang.org/x/mobile/bind/java"
+	"golang.org/x/mobile/bind/java"
 	_ "golang.org/x/mobile/example/libhello/hi/go_hi"
 )
 
 func main() {
-	app.Run(app.Callbacks{})
+	app.Run(app.Callbacks{Start: java.Init})
 }