Make registration ID optionally extended.

libaxolotl/src/main/java/org/whispersystems/libaxolotl/util/KeyHelper.java

@@ -38,11 +38,18 @@ public class KeyHelper {
    * Generate a registration ID.  Clients should only do this once,
    * at install time.
    *
+   * @param extendedRange By default (false), the generated registration
+   *                      ID is sized to require the minimal possible protobuf
+   *                      encoding overhead. Specify true if the caller needs
+   *                      the full range of MAX_INT at the cost of slightly
+   *                      higher encoding overhead.
    * @return the generated registration ID.
    */
-  public static int generateRegistrationId() {
+  public static int generateRegistrationId(boolean extendedRange) {
     try {
-      return SecureRandom.getInstance("SHA1PRNG").nextInt(16380) + 1;
+      SecureRandom secureRandom = SecureRandom.getInstance("SHA1PRNG");
+      if (extendedRange) return secureRandom.nextInt(Integer.MAX_VALUE - 1) + 1;
+      else               return secureRandom.nextInt(16380) + 1;
     } catch (NoSuchAlgorithmException e) {
       throw new AssertionError(e);
     }