Simplify passwordToHistoryHash()

When processing the password and salt, just use update() instead of
concatenating them into an array.  No change in behavior, as verified by
LockscreenCredentialTest.testPasswordToHistoryHash().

Test: atest LockscreenCredentialTest
Change-Id: I92e240dbe4df6c30fe35444b1452bb003421243b

core/java/com/android/internal/widget/LockscreenCredential.java

@@ -278,11 +278,8 @@ public class LockscreenCredential implements Parcelable, AutoCloseable {
         try {
             MessageDigest sha256 = MessageDigest.getInstance("SHA-256");
             sha256.update(hashFactor);
-            byte[] saltedPassword = Arrays.copyOf(passwordToHash, passwordToHash.length
-                    + salt.length);
-            System.arraycopy(salt, 0, saltedPassword, passwordToHash.length, salt.length);
-            sha256.update(saltedPassword);
-            Arrays.fill(saltedPassword, (byte) 0);
+            sha256.update(passwordToHash);
+            sha256.update(salt);
             return new String(HexEncoding.encode(sha256.digest()));
         } catch (NoSuchAlgorithmException e) {
             throw new AssertionError("Missing digest algorithm: ", e);