Hide WriteableIdentityCredential constructor from public API

Bug: 111446262
Test: CtsIdentityTestCases
Change-Id: Iafe8e76e6491ff92ee751702b8fb44aeda7355a8

api/current.txt

@@ -41189,7 +41189,6 @@ package android.security.identity {
   }
 
   public abstract class WritableIdentityCredential {
-    ctor public WritableIdentityCredential();
     method @NonNull public abstract java.util.Collection<java.security.cert.X509Certificate> getCredentialKeyCertificateChain(@NonNull byte[]);
     method @NonNull public abstract byte[] personalize(@NonNull android.security.identity.PersonalizationData);
   }

identity/java/android/security/identity/WritableIdentityCredential.java

@@ -30,6 +30,11 @@ import java.util.Collection;
  * Use {@link IdentityCredentialStore#createCredential(String, String)} to create a new credential.
  */
 public abstract class WritableIdentityCredential {
+    /**
+     * @hide
+     */
+    protected WritableIdentityCredential() {}
+
     /**
      * Generates and returns an X.509 certificate chain for the CredentialKey which identifies this
      * credential to the issuing authority. The certificate contains an