Make DexString constructor private to only allow new in DexItemFactory

Change-Id: I0eeb8d4b6ae24edcc8666bc44a176c8fd70484c0

src/main/java/com/android/tools/r8/graph/DexString.java

diff --git a/src/main/java/com/android/tools/r8/graph/DexString.java b/src/main/java/com/android/tools/r8/graph/DexString.java
index b591bab..a89ff39 100644
--- a/src/main/java/com/android/tools/r8/graph/DexString.java
+++ b/src/main/java/com/android/tools/r8/graph/DexString.java
@@ -23,7 +23,7 @@
     this.content = content;
   }
 
-  public DexString(String string) {
+  DexString(String string) {
     this.size = string.length();
     this.content = encodeToMutf8(string);
   }