Hide non-system overlay window on ActivityPicker

To improve security.

Bug: 181962311
Test: manual
      Show an AlertDialog and observe if it will hide after below command.
      adb shell am start -a android.intent.action.PICK_ACTIVITY -n com.android.settings/.ActivityPicker
Change-Id: I6e2845cc19dc012cba2933318a067bbb8db90a23
(cherry picked from commit 3b4853e109)

src/com/android/settings/ActivityPicker.java

@@ -16,6 +16,8 @@
 
 package com.android.settings;
 
+import static android.view.WindowManager.LayoutParams.SYSTEM_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS;
+
 import android.app.Activity;
 import android.content.Context;
 import android.content.DialogInterface;
@@ -72,6 +74,8 @@ public class ActivityPicker extends AlertActivity implements
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
+        getWindow().addPrivateFlags(SYSTEM_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS);
+        
         final Intent intent = getIntent();
         
         // Read base intent from extras, otherwise assume default