Added PRIVATE_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS window flag.

When a window with this flag is visible, then any other window added
by an application process that requires OP_SYSTEM_ALERT_WINDOW
permission or that are of TYPE_TOAST will be hidden.

Bug: 62196835
Test: TODO
Change-Id: I07605b52af893945250b0212f406e0e92bfd92bd