main/postmarketos-ui-phosh: disable auto-brightness by default (MR 1348)

We should disable it by default and let the user to enable it
when they want it.

And currently it's causing problems where the backlight would
go very dark with it enabled on PinePhone.

Signed-off-by: Danct12 <danct12@disroot.org>
(cherry picked from commit f7ed67b1)