Disabled snap timer service is re-enabled upon refresh or reboot

[Alex Zorin]

To my understanding, if the user disables a snap's timer service, it should persist through snap refreshes and reboots. This was discussed at https://forum.snapcraft.io/t/cant-mask-or-disable-snap-services-why-etc-systemd-instead-of-lib-systemd/33385, leading to this bug report.

It seems that both rebooting and refreshing the snap causes the timer to be re-enabled. Reproductions follow.

Case 1: Disabling the timer and rebooting, re-enables the timer:
---

  # snap install --classic certbot

  # systemctl status snap.certbot.renew.timer
  ● snap.certbot.renew.timer - Timer renew for snap application certbot.renew
       Loaded: loaded (/etc/systemd/system/snap.certbot.renew.timer; enabled; vendor preset: enabled)
       Active: active (waiting) since Thu 2023-01-12 07:36:25 AEDT; 49s ago
      Trigger: Thu 2023-01-12 21:55:00 AEDT; 14h left
     Triggers: ● snap.certbot.renew.service

  # snap stop --disable certbot
  Stopped.

  # snap services certbot
  Service Startup Current Notes
  certbot.renew enabled inactive timer-activated

  # systemctl status snap.certbot.renew.timer
  ○ snap.certbot.renew.timer - Timer renew for snap application certbot.renew
      Loaded: loaded (/etc/systemd/system/snap.certbot.renew.timer; enabled; vendor preset: enabled)
      Active: inactive (dead) since Thu 2023-01-12 07:39:03 AEDT; 2min 4s ago
      Trigger: n/a
    Triggers: ● snap.certbot.renew.service

  # reboot

  # snap services certbot
  Service Startup Current Notes
  certbot.renew enabled inactive timer-activated

  # systemctl list-timers --all | grep -i certbot
  Thu 2023-01-12 08:38:00 AEDT 54min left n/a n/a snap.certbot.renew.timer snap.certbot.renew.service

Case 2: Disabling the timer and refreshing it, re-enables the timer:
---

  # snap install --classic certbot

  # systemctl list-timers --all | grep -i certbot
  Thu 2023-01-12 08:38:00 AEDT 51min left n/a n/a snap.certbot.renew.timer snap.certbot.renew.service

  # snap stop --disable certbot
  Stopped.

  # systemctl list-timers --all | grep -i certbot
  n/a n/a n/a n/a snap.certbot.renew.timer snap.certbot.renew.service

  # snap refresh --beta certbot
  certbot (beta) 2.1.1 from Certbot Project (certbot-eff✓) refreshed

  # systemctl list-timers --all | grep -i certbot
  Thu 2023-01-12 19:37:00 AEDT 11h left n/a n/a snap.certbot.renew.timer snap.certbot.renew.service

  # snap services certbot
  Service Startup Current Notes
  certbot.renew enabled inactive timer-activated

Versions
---

  # cat /etc/os-release
  PRETTY_NAME="Ubuntu 22.04.1 LTS"
  NAME="Ubuntu"
  VERSION_ID="22.04"
  VERSION="22.04.1 LTS (Jammy Jellyfish)"
  VERSION_CODENAME=jammy
  ID=ubuntu
  ID_LIKE=debian
  HOME_URL="https://www.ubuntu.com/"
  SUPPORT_URL="https://help.ubuntu.com/"
  BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
  PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
  UBUNTU_CODENAME=jammy

  # snap version
  snap 2.58
  snapd 2.58
  series 16
  ubuntu 22.04
  kernel 5.15.0-1026-oracle