Need to debounce multiple rfkill events.

[khfeng]

 * csd version - 4.8.5-1
 * Distribution - Ubuntu Impish Indri
 * Graphics hardware *and* driver used - Intel and i915
 * 32 or 64 bit - 64bit

Issue
Newer HP laptops use Intel HID driver to send rfkill hotkey event, so we'll need to unmask Intel HID in hwdb to make airplane mode hotkey work:
systemd/systemd#20219
However, this will make both Intel HID driver and HP Wireless driver send rfkill events on older HP laptops, so only the first event like should be processed, like this implementation in G-S-D:
https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/merge_requests/244
The current behavior is that WiFi and Bluetooth get quickly disabled and enabled after pressing airplane mode hotkey.

Steps to reproduce

1. Apply the change in systemd/systemd#20219
2. Press airplane mode hotkey.
3. WiFi and Bluetooth gets disabled then enabled immediately.

Expected behaviour
WiFi and Bluetooth should be disabled.

Other information
Seems like the entire rfkill part is missing in C-S-D, so I can't do a straight backport from G-S-D.