is_paytm_activated shouldn't be a read only attribute

[mrsaicharan1]

Describe the bug
Alipay, Omise, and PayPal have a property to enable them(e.g. is_paypal_activated). They were hybrid properties(condition was like if client_key and secret_key then true), so they had a dump_only=True. We need to remove this as we've moved away from such a practice. The reason for this is because we need the keys to be stored, whether the property is enabled/disabled

Additional context
Working on this.