Remove special handling of recents

.. now that they've moved over to use SYSTEM_APPLICAITON_OVERLAY instead
of SYSTEM_ALERT_WINDOW.

Also removes recents from the protectionLevel of SAW.

Bug: 169054709
Test: atest PermissionPolicyTest
Test: atest CtsWindowManagerDeviceTestCases:HideOverlayWindowsTest
Test: Manually verified with modified launcher that recent section is
still visible while browsing settings

Change-Id: Ie8e21158d6c12eb43c5411d0cb2259a8823ccacd