Add hide-non-system-overlay flag for HarmfulAppWarningActivity

A malicious application could overlay the activity. The overlay is
able to be tapped through, which can trick the user into starting a
harmful activity.

The CL added the flag SYSTEM_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS for
the activity to prevent the tapjacking/overlay attack.

Bug: 205595291
Test: atest CtsHarmfulAppWarningHostTestCases
Change-Id: Ia1a1ae0dc451e04bf5c31e3cb8cf30a0d8e32991