Custom Hints, Button Pay Text and Fixed App Crashes

[CristianRP]

Features added:

• Custom hint text for card name, card number, month, year and cvv input texts.

• Custom text for button pay

Fixes:

• The app crash when activity was initialized and push the button pay, that was for a NumberFormatException on the input of cvv, month and year. To fix that the validations was modified and was added a isNullOrEmpty before get text string.

Also the kotlin version and gradle was updated.