Fix Clear Credentials item

This should be enabled whenever there is a credential in the keystore
since there can be unencrypted credentials in there now.

(cherry picked from commit c4372c3e3f)

Bug: 8651830
Change-Id: Ie06f20cf90f5a359975cb30d67052f8867b429b9

src/com/android/settings/SecuritySettings.java

@@ -415,7 +415,7 @@ public class SecuritySettings extends SettingsPreferenceFragment
         }
 
         if (mResetCredentials != null) {
-            mResetCredentials.setEnabled(!mKeyStore.isUnlocked());
+            mResetCredentials.setEnabled(!mKeyStore.isEmpty());
         }
     }