Remove appdatabase get instance

[TimoPtr]

Summary

Remove the need of AppDatabase.getInstance and removing the synchronization block from it. It might help reduce some ANR.

Checklist

• New or updated tests have been added to cover the changes following the testing guidelines.
• The code follows the project's code style and best_practices.
• The changes have been thoroughly tested, and edge cases have been considered.
• Changes are backward compatible whenever feasible. Any breaking changes are documented in the changelog for users and/or in the code for developers depending on the relevance.

Any other notes

Closes #5702

[jpelgrom]

Sometimes you're using val dao = ..., sometimes it is val sensorDao = ... (example). Try to be consistent. My preference would be to use a specific name (sensorDao), reducing changed lines, or at least use the specific name if any other DAO is accessed in the same file to prevent mixups.