mobile/bind: initialize imported bound packages

Make sure that a bound package's imported and also bound packages
are initialized before referencing methods and constructors in the
imported packages.

Change-Id: If158aac83c245a33695d3b1648d0dfc37a7313ac
Reviewed-on: https://go-review.googlesource.com/20652
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/testdata/structs.java.golden

@@ -14,6 +14,9 @@ public abstract class Structs {
     
     private Structs() {} // uninstantiable
     
+    // touch is called from other bound packages to initialize this package
+    public static void touch() {}
+    
     private static native void init();
     
     public static final class S implements go.Seq.Object {