[Cronet] Separate Cronet implementation and API by package name.

components/cronet/android/api/src/org/chromium/net/Preconditions.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.net;
 
 import java.nio.ByteBuffer;
 
-final class Preconditions {
+/**
+ * {@hide only used by internal implementation.}
+ */
+public final class Preconditions {
     private Preconditions() {}
 
-    static void checkDirect(ByteBuffer buffer) {
+    public static void checkDirect(ByteBuffer buffer) {
         if (!buffer.isDirect()) {
             throw new IllegalArgumentException("byteBuffer must be a direct ByteBuffer.");
         }
     }
 
-    static void checkHasRemaining(ByteBuffer buffer) {
+    public static void checkHasRemaining(ByteBuffer buffer) {
         if (!buffer.hasRemaining()) {
             throw new IllegalArgumentException("ByteBuffer is already full.");
         }
     }
 }