Allow setup app to have SAW permission

Why not SYSTEM_APPLICATION_OVERLAY: The setup app shouldn't be able to
show an overlay on top of apps that don't consent to it.

Change-Id: I8a9d6d409ab1082bc36e4ffd220632a501dc2dca
Test: adb shell dumpsys permission | grep -C10 SYSTEM_ALERT_WINDOW
Test: atest CtsPermission2TestCases
Bug: 182471350