pallet-contracts: State rent fixes (#6147)

* Don't store the storage size offset in the contract itself.

* Clean the AccountDb code a bit

* Use `storage_size: 0` when creating AliveContractInfo

* Count empty storage items.

* Update frame/contracts/src/account_db.rs

Co-authored-by: Nikolay Volf <[email protected]>

* Use more clear wording.

Co-authored-by: Alexander Theißen <[email protected]>

* Change the order of decrement and increment for storage size

Co-authored-by: Nikolay Volf <[email protected]>
Co-authored-by: Alexander Theißen <[email protected]>