Move PreKey ids to be Mediums, generate in circular buffer.

library/src/org/whispersystems/textsecure/util/Medium.java

@@ -0,0 +1,5 @@
+package org.whispersystems.textsecure.util;
+
+public class Medium {
+  public static int MAX_VALUE = 0xFFF;
+}

library/src/org/whispersystems/textsecure/util/Util.java

@@ -119,4 +119,12 @@ public class Util {
 
     return result.toString();
   }
+
+  public static SecureRandom getSecureRandom() {
+    try {
+      return SecureRandom.getInstance("SHA1PRNG");
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    }
+  }
 }