fix: resolve `org.dmfs.tasks.model.ModelInflaterException` generated by Tasks app

Tasks app is looking for a model definition XML file from the Account Manager app but can't find it because the Account Manager app doesn't provide the required org.dmfs.tasks.TASKS metadata.

To fix the exception in Account Manager:
1. Added the org.dmfs.tasks.TASKS metadata to the OpenTasksSyncAdapterService in the AndroidManifest.xml
2. Created the tasks_model.xml file with the appropriate task field definitions
3. Added the required string resources