Set device credential's Window flag to be SECURE.

This prevents showing any pattern/pin in recent list.

Change-Id: Ife33131004d66092bfa57aa57a9bb856800ce28d
Fixes: 64339515
Bug: 38258991
Merged-In: Ia9935ef710cf1795d9ce464e0a9edff75be9c0c3
Merged-In: I99c283dd4a1f2d579f565ccf5839a705e75ed6fc
Merged-In: Ibe781a3a1e1483a8c6fde4c7cc5dffbf54bdba59
Test: manual

src/com/android/settings/ConfirmDeviceCredentialBaseActivity.java

@@ -45,6 +45,7 @@ public abstract class ConfirmDeviceCredentialBaseActivity extends SettingsActivi
             mDark = true;
         }
         super.onCreate(savedState);
+        getWindow().addFlags(WindowManager.LayoutParams.FLAG_SECURE);
         mIsKeyguardLocked = savedState == null
                 ? getSystemService(KeyguardManager.class).isKeyguardLocked()
                 : savedState.getBoolean(STATE_IS_KEYGUARD_LOCKED, false);