feat: Legacy XprvDouble Support

[parodyBit]

the test matches the one here:
https://github.com/witnet/witnet-rust/blob/master/wallet/src/actors/app/tests/mod.rs#L185-L198

[Tommytrg]

Could you run dart format .?

[Tommytrg]

Could we rename the method fromEncryptedXprv with fromEncryptedXprvSingle and then create a method fromEncryptedXprv that receives an XPRV and a password and calls fromEncryptedXprv orfromEncryptedXprvSingle according to the length of the XPRV?

const ENCRYPTED_XPRV_DOUBLE_LENGTH = 478;
const ENCRYPTED_XPRV_SINGLE_LENGTH = 293;

static List<Xprv> fromEncryptedXprv(String xprv, String password) {
  try {
    int xprvLength = xprvString.length;
    if (xprvLength == ENCRYPTED_XPRV_SINGLE_LENGTH)  {
      return Xprv.fromEncryptedXprvSingle(xprv, password); 
    }

    if (xprvLength == ENCRYPTED_XPRV_DUBLE_LENGTH)  {
       return Xprv.fromEncryptedXprvDouble(xprv, password); 
    }      

    if (xprvLength != ENCRYPTED_XPRV_DUBLE_LENGTH && xprvLength != ENCRYPTED_XPRV_SINGLE_LENGTH)  {
      throw proper_error_because_xprv_is_invalid();
    }
  } catch (e) {
     rethrow;
  }
}