mobile/bind: implement interfaces from imported bound packages

Java classes must explicitly declare implemented interfaces. Bind
already declares all such interfaces within each package. Expand
the set of interfaces to include all bound packages.

In addition, let Java interfaces extend all possible interfaces in
the same way as Java classes. To avoid circular references, only
let interfaces extend compatible interfaces with fewer methods.

Before, each package was imported in its own importer, breaking the
assumption of types.AssignableTo that identical packages have
identical *types.Package. Fix that by using one importer for all
bound packages, replacing package path equality checks with direct
equality checks.

While we're here, add missing arguments to a few error messages.

Change-Id: I5eb58972a3abe918862ca99d5a203809699a3433
Reviewed-on: https://go-review.googlesource.com/20987
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/SeqTest.java

@@ -496,6 +496,10 @@ public class SeqTest extends InstrumentationTestCase {
     fields.setS(s);
     Testpkg.WithImportedI(i);
     Testpkg.WithImportedS(s);
+
+    Secondpkg.IF f = new AnI();
+    f = Testpkg.New();
+    Secondpkg.Ser ser = Testpkg.NewSer();
   }
 
   public void testIDup() {