Prevent screen capture for Desktop Backup Password

Bug: 254028772
Change-Id: I9c75398413f0f4a92d7a8f11d1e0b8d1e26e2961
Test: manual

src/com/android/settings/SetFullBackupPassword.java

@@ -25,6 +25,7 @@ import android.text.TextUtils;
 import android.util.Log;
 import android.view.View;
 import android.view.View.OnClickListener;
+import android.view.WindowManager;
 import android.widget.Button;
 import android.widget.TextView;
 import android.widget.Toast;
@@ -80,6 +81,7 @@ public class SetFullBackupPassword extends Activity {
     @Override
     public void onCreate(Bundle icicle) {
         super.onCreate(icicle);
+        getWindow().addFlags(WindowManager.LayoutParams.FLAG_SECURE);
 
         mBackupManager = IBackupManager.Stub.asInterface(ServiceManager.getService("backup"));