Set default float brightness constraint to invalid

The default setting is now invalid, so that if there is an overlay, it
is used and if not, the default int value is used.

Bug: b/149665432

Test: manual testing - ensure that the brightness value in
Settings>Display>Brightness Level can go the full range from 0% to 100%
(on a device without a float value in the overlay).

Change-Id: I52f3a66e33914bb91e05a6ae90f75d26dc002046