Settings: add wifi ap dialog 'show password' default overlay

Allow specifying a default value for whether "Show password" is
preselected in the WiFi tethering setup dialog. Also save the state and
restore it if the user changes the value.

Change-Id: Ib2ea8c1831f1c2cbef16dbf649624a24cc06133b
Signed-off-by: Roman Birg <roman@cyngn.com>