fix : Invalid values are accepted for the date of birth of a customer

[AbhilashG97]

Please make sure these boxes are checked before submitting your pull request - thanks!

Fixes #175

• [x] Apply the AndroidStyle.xml style template to your code in Android Studio.

• [x] Run the unit tests with ./gradlew check to make sure you didn't break anything

• [x] If you have multiple commits please combine them into one commit by squashing them.

After making the change-

[AbhilashG97]

@therajanmaurya, please let me know if I have to make any further changes.