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: I43bb0f47a96719c61c5beb4ddf486b14cbdd6ee8
Merged-In: I6e2845cc19dc012cba2933318a067bbb8db90a23
(cherry picked from commit 636e70fb)