Reading time format from the settings portal during context init results
in interacting with xdg-desktop-portal before GtkApplication can
register the host app id. This makes portals not aware of the
application id, which could cause problems in the future.
Fix that by calling read_time_format during gcal_context_startup, which
is called after the GtkApplication startup chain-up.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/586>
