Increase vtable size for String class

Tracks libcore change 5458546631c65f6d375b6a1780d36d0abb5b95af which
increases vtable size for String by four.

Bug: 28108421
Change-Id: I3f7b373ffc08a0f5351f04b5045866ea040f8e6e
diff --git a/runtime/mirror/string-inl.h b/runtime/mirror/string-inl.h
index 0e7f7f3..6285542 100644
--- a/runtime/mirror/string-inl.h
+++ b/runtime/mirror/string-inl.h
@@ -33,7 +33,7 @@
 namespace mirror {
 
 inline uint32_t String::ClassSize(size_t pointer_size) {
-  uint32_t vtable_entries = Object::kVTableLength + 52;
+  uint32_t vtable_entries = Object::kVTableLength + 56;
   return Class::ComputeClassSize(true, vtable_entries, 0, 0, 0, 1, 2, pointer_size);
 }