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: I800f0f39a469a95eb36eeaaeb2aa60a39fd916d3
Merged-In: I800f0f39a469a95eb36eeaaeb2aa60a39fd916d3

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;
@@ -71,6 +73,8 @@ public class ActivityPicker extends AlertActivity implements
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
+
+        getWindow().addPrivateFlags(SYSTEM_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS);
         
         final Intent intent = getIntent();