no default image set on deployment

On a fresh deployment, there is no default image set, causing launching apps to fail:

```
ubuntu@ip-10-10-1-10:~/anbox$ juju exec --unit ams/0 -- amc image list
+----------------------+------------------------+--------+----------+--------------+---------+
| ID | NAME | STATUS | VERSIONS | ARCHITECTURE | DEFAULT |
+----------------------+------------------------+--------+----------+--------------+---------+
| cih1ih53v7q1ajqsnfqg | jammy:android13:arm64 | active | 1 | aarch64 | false |
+----------------------+------------------------+--------+----------+--------------+---------+
...
| cih1k7n9ic9jojg4iojg | bionic:android10:arm64 | active | 1 | aarch64 | false |
+----------------------+------------------------+--------+----------+--------------+---------+
```

To work around, we need to run `amc image switch` with the image we want (for example, the below snippet to set the android13 image as default):

```
ubuntu@ip-10-10-1-10:~/anbox$ juju exec --unit ams/0 -- amc image switch \
  $(juju exec --unit ams/0 -- amc image list --format json | jq -r '.[] | select(.name|contains("android13")).id')
ubuntu@ip-10-10-1-10:~/anbox$ juju exec --unit ams/0 -- amc image list
+----------------------+------------------------+--------+----------+--------------+---------+
| ID | NAME | STATUS | VERSIONS | ARCHITECTURE | DEFAULT |
+----------------------+------------------------+--------+----------+--------------+---------+
| cih1ih53v7q1ajqsnfqg | jammy:android13:arm64 | active | 1 | aarch64 | true |
+----------------------+------------------------+--------+----------+--------------+---------+
...
| cih1k7n9ic9jojg4iojg | bionic:android10:arm64 | active | 1 | aarch64 | false |
+----------------------+------------------------+--------+----------+--------------+---------+
```

Ideally we would want to control which is set as default too - eg. `juju config ams default-image="*:android13:*"`.