Configurable drop down answer option.

[AdityaKBhadragond14]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2505

Description In the DropDownViewHolderFactory the hyphen("-") is being set as the default answer option to the drop down. Instead of setting it a fixed answer I have made it configurable. To change the default answer option the questionnaire item representing the drop down item will have the initial property which will have a valueString. The default answer option will be filled with the initial property value if available else it will be filled hyphen("-"). This approach ensures that the configuration will be done via questionnaire itself and the code need be changed everytime to change a default answer option.

Alternative(s) considered Have you considered any alternatives? And if so, why have you chosen the approach in this PR?

Type Bug Fix.

Screenshots (if applicable)

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