When importing a watch-only address over `importmulti` with a specific timestamp, the wallet's `nTimeFirstKey` will be set to 1.
I think we should add a optional timestamp to `CWallet::AddWatchOnly()` and use it when importing addresses or pubkeys.

{
    AssertLockHeld(cs_wallet); 
    UpdateTimeFirstKey(meta.nCreateTime);
    mapKeyMetadata[pubkey.GetID()] = meta;
    return true;
}
bool CWallet::LoadCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret)
{
    return CCryptoKeyStore::AddCryptedKey(vchPubKey, vchCryptedSecret);
}
void CWallet::UpdateTimeFirstKey(int64_t nCreateTime)
{
    AssertLockHeld(cs_wallet);
    if (nCreateTime <= 1) {
        
        
        nTimeFirstKey = 1;
    } else if (!nTimeFirstKey || nCreateTime < nTimeFirstKey) {
        nTimeFirstKey = nCreateTime;
    }
}
bool CWallet::AddCScript(const CScript& redeemScript)