bind,internal: change the default Java package to the empty string

The Objective-C bindings was recently changed to support the empty
name prefix and to use that as the default. This CLs changed the Java
generators in the same way, supporting the empty Java package and using
it as the default.

Change-Id: I857affce686c67638a2b6c4e1da5d6a88d7ba560
Reviewed-on: https://go-review.googlesource.com/34778
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/SeqBench.java

@@ -13,7 +13,7 @@ import java.util.HashMap;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ExecutorService;
 
-import go.benchmark.*;
+import benchmark.*;
 
 public class SeqBench extends InstrumentationTestCase {
 
@@ -22,7 +22,7 @@ public class SeqBench extends InstrumentationTestCase {
     }
   }
 
-  private static class Benchmarks implements go.benchmark.Benchmarks {
+  private static class Benchmarks implements benchmark.Benchmarks {
     private static Map<String, Runnable> benchmarks;
     private static ExecutorService executor = Executors.newSingleThreadExecutor();