Period setting is not kept when going back to the settings form

Problem/Motivation

Steps to reproduce

After enabling the matomo_reporting_api_example module, we can configure the block that is displayed and choose the "Past week period".
However when going back to the settings form, the period is lost.

Proposed resolution

This is due to an incomplete form missing the #default_value, so let's add it.

Remaining tasks

Create Merge Request.

User interface changes

None

API changes

None

Data model changes