Removing a snap triggers 'Starting scheduled backup' notification

[Ads20000]

Removing a snap triggers the 'Starting scheduled backup' notification then (because I don't have the USB stick plugged in) the `Backup Failed: Error creating directory /media/adam/Adam Eveleigh: Permission denied` dialogue box. I am scheduled a backup, but should this dialogue box be opened every time I remove a snap?

`journalctl -f` output for `snap remove notepad-plus-plus`: https://paste.ubuntu.com/p/Ksy4cmF6ZJ/

```
$ snap version
snap 2.36.3+git1065.557f8c3~ubuntu16.04.1
snapd 2.36.3+git1065.557f8c3~ubuntu16.04.1
series 16
ubuntu 18.10
kernel 4.18.0-13-generic
$ snap info core
tracking: edge
refresh-date: today at 11:20 GMT
channels:
  stable: 16-2.36.3 (6130) 93MB -
  candidate: 16-2.36.3 (6130) 93MB -
  beta: 16-2.36.3 (6130) 93MB -
  edge: 16-2.36.3+git1065.557f8c3 (6199) 95MB -
installed: 16-2.36.3+git1065.557f8c3 (6199) 95MB core
```

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: deja-dup 38.0-1
ProcVersionSignature: Ubuntu 4.18.0-13.14-generic 4.18.17
Uname: Linux 4.18.0-13-generic x86_64
NonfreeKernelModules: binder_linux ashmem_linux
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Tue Dec 25 11:24:59 2018
InstallationDate: Installed on 2017-08-03 (508 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: deja-dup
UpgradeStatus: Upgraded to cosmic on 2018-10-21 (65 days ago)

[Ads20000 (ads20000)]

I think this is possibly a Deja Dup bug (so I used `ubuntu-bug deja-dup`) but I've marked it as also affecting snappy for now because it's an interaction bug so might be snappy that needs to be fixed rather than Deja Dup.

[Zygmunt Krynicki (zyga)]

I suspect this is caused by `udevadm trigger` and `udevadm settle` that snapd invokes, for some snaps, depending on how they are made, when they are removed.

There are multiple other bugs related to various issues (including desktop crashing or minor annoyances) related to udev. I've added a tag to this bug to ease discovery of such bugs for other developers.