Remove pallet::getter usage from pallet-contracts-mock-network (#4417)

A part of #3326 

Removes all #[pallet::getter] usage from the contracts mock network
pallet. As the storage values were pub(super), read-only visibility was
lost external to the crate upon the removal of the macros. I have
implemented custom getters as a replacement, keeping the api the same.

If we care very much about consistency of the
storagevalue::<T>::get() syntax, the other option would be to set
the storage values to pub. Though I find preserving data authority
better myself.

@muraca