Protect GrantCredentialsPermissionActivity against overlay.

Bug: 169763814
Test: manual
Change-Id: I15dd22791fcc61ef02b06ad51d9e4409d11c0181
diff --git a/core/java/android/accounts/GrantCredentialsPermissionActivity.java b/core/java/android/accounts/GrantCredentialsPermissionActivity.java
index af74b03..32b61b5 100644
--- a/core/java/android/accounts/GrantCredentialsPermissionActivity.java
+++ b/core/java/android/accounts/GrantCredentialsPermissionActivity.java
@@ -47,6 +47,9 @@
 
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
+        getWindow().addSystemFlags(
+                android.view.WindowManager.LayoutParams
+                        .SYSTEM_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS);
         setContentView(R.layout.grant_credentials_permission);
         setTitle(R.string.grant_permissions_header_text);