Disabled previous dates in the date picker

Abhinavrequested to merge
abhinav12k/agora-android:calender-start-date into develop

Fixes #290 (closed)

Appetize Link - https://appetize.io/app/4axncnxykmgqnwn0ee26mqp188

Changes :
Added min date to the date picker

Screenshots for the change :
Screenshot_20210317-132928_Agora_Vote

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