rfkill: Delay writes until exit (#5768)

On thinkpads there are two rfkill devices for bluetooth. The first is an
ACPI switch which powers down the USB dongle and the second one is the
USB dongle itself. So when userspace decides to enable rfkill on all
devices systemd would randomly save the soft block state of the USB
dongle. This later causes issue when re-enabling the devie as
systemd-rfkill would put the USB dongle into soft block state right
after the ACPI rfkill switch is unblocked by userspace.

The simple way to avoid this is to not store rfkill changes for devices
that disappear shortly after. That way only the "main" ACPI switch will
get stored and systemd-rfkill will not end up blocking the device right
after it is being added back again.