Fix BitVector::IndexIterator::operator*() to return uint32_t.

Change-Id: I3cfc028b1c3744ec85ea00eadcbccfdde6fd51d3
diff --git a/runtime/base/bit_vector.h b/runtime/base/bit_vector.h
index 1e28a27..557a2ec 100644
--- a/runtime/base/bit_vector.h
+++ b/runtime/base/bit_vector.h
@@ -52,7 +52,7 @@
       return !(*this == other);
     }
 
-    int operator*() const;
+    uint32_t operator*() const;
 
     IndexIterator& operator++();