Add HmacSHA256 backed by AndroidKeyStore.

This also adds the MAC length constraint on imported HMAC keys. HMAC
doesn't work without this constraint at the moment.

Bug: 18088752
Change-Id: I8613f58f5d2a84df00bcf6179d13e30619440330

keystore/java/android/security/KeymasterException.java

@@ -7,7 +7,14 @@ package android.security;
  */
 public class KeymasterException extends Exception {
 
-    public KeymasterException(String message) {
+    private final int mErrorCode;
+
+    public KeymasterException(int errorCode, String message) {
         super(message);
+        mErrorCode = errorCode;
+    }
+
+    public int getErrorCode() {
+        return mErrorCode;
     }
 }