example/libhellojni: fix build and allow go get golang.org/x/mobile/...

Change-Id: Id0cd58705c882ea58cedbee5a6e9aff010c0ad4d
Reviewed-on: https://go-review.googlesource.com/1281
Reviewed-by: Andrew Gerrand <adg@golang.org>

bind/java/doc.go

@@ -0,0 +1,6 @@
+// Package java implements the Java language bindings.
+//
+// See the design document (http://golang.org/s/gobind).
+//
+// Currently, this works only for android.
+package java

example/libhellojni/hello.c

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build android
+
 // See main.go for commentary.
 
 #include <android/log.h>

example/libhellojni/hello.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build android
+
 package main
 
 // #cgo LDFLAGS: -llog

example/libhellojni/main.go

@@ -26,5 +26,5 @@ package main
 import "golang.org/x/mobile/app"
 
 func main() {
-	app.Run()
+	app.Run(app.Callbacks{})
 }