mobile/bind: fix gomobile bind with custom Java package

The change from using strings to objects for passing errors across
the language barrier broke the custom java package mode of gombile
bind. Fix it and add a runtime test to make sure it won't happen
again.

Fixes golang/go#16262

Change-Id: Ia7f8afb79556798056f0755758052190081a2dbb
Reviewed-on: https://go-review.googlesource.com/24800
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/CustomPkgTest.java

@@ -0,0 +1,15 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package go;
+
+import android.test.InstrumentationTestCase;
+
+import org.golang.custompkg.Testpkg;
+
+public class CustomPkgTest extends InstrumentationTestCase {
+  public void testHi() {
+    Testpkg.Hi();
+  }
+}