Revert^2 "Remove hidden System.arraycopy(byte[]...."

This reverts commit 5857c9995cf66027e901c87980a7b0d2a3bb4e63.

Reason for revert: The problems with certain prebuilts that were still referencing the non-public / hidden method should now be dealt with under bug 130207940

Original description:

Remove hidden System.arraycopy(byte[]....

This method is hidden / not part of the public SDK API. Nobody should
be using it.

Bug: 128267537
Bug: 74103559
Change-Id: Id1b1786feb4ea114962d395f7c6beb5faf71a7e7
Test: build only
diff --git a/ojluni/src/main/java/java/lang/System.java b/ojluni/src/main/java/java/lang/System.java
index 8235bf7..d698446 100644
--- a/ojluni/src/main/java/java/lang/System.java
+++ b/ojluni/src/main/java/java/lang/System.java
@@ -509,13 +509,9 @@
      * The byte[] specialized version of arraycopy().
      * Note: This method is required for runtime ART compiler optimizations.
      * Do not remove or change the signature.
-     * Note: Unlike the others, this variant is public due to a dependency we
-     * are working on removing. b/74103559
-     *
-     * @hide
      */
     @SuppressWarnings("unused")
-    public static void arraycopy(byte[] src, int srcPos, byte[] dst, int dstPos, int length) {
+    private static void arraycopy(byte[] src, int srcPos, byte[] dst, int dstPos, int length) {
         if (src == null) {
             throw new NullPointerException("src == null");
         }