Ensure Direct Calls on set_key

[bh2smith]

As discussed in #13 (comment) we should be referring to predecessor account id here instead of signer account. I thought about it a bit more and it seemed as if using predecessor account id could also result in undesired behaviour. Instead I went with maximal security decision to ensure this method can only be called by account holder directly.

1. Using SignerAccountId: could result in accountA signing a contract execution on accountB which internally makes a cross-contract call to set_key and thus overwriting accountA's key.

2. Using PredecessorAccountId: Means that the same a user would not necessarily be able to use a contract to set their own key, since they are transacting through an intermediary contract with a different ID.

⚠️ I suppose the argument for using predecessor_account_id here is in case walletA has multiple signers and one of those signers would like to set the key for walletA, then signer_i would want to execute set_key via walletA contract. I guess I will have to change this.