OmemoManagerPrivate: Fix device setup

[melvo]

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [x] Update doc/doap.xml
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Base: 68.36% // Head: 68.34% // Decreases project coverage by -0.02% :warning:

Coverage data is based on head (16b4167) compared to base (cbf3209). Patch coverage: 0.00% of modified lines in pull request are covered.

:exclamation: Current head 16b4167 differs from pull request most recent head c50784a. Consider uploading reports for the commit c50784a to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #507 +/- ## ========================================== - Coverage 68.36% 68.34% -0.03% ========================================== Files 298 298 Lines 25869 25877 +8 ========================================== Hits 17685 17685 - Misses 8184 8192 +8 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/507?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project) | Coverage Δ | | |---|---|---| | [src/omemo/QXmppOmemoManager\_p.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/507/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL29tZW1vL1FYbXBwT21lbW9NYW5hZ2VyX3AuY3Bw) | `4.98% <0.00%> (-0.03%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project)

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[melvo]

This change is especially needed in order to set up OMEMO with Prosody.