Add KeyChainGroup.isDeterministic() and Wallet.isDeterministic(). Add static constructor for non-HD wallets.

Paper wallets are non-HD. That's what newNonDeterministicWallet() is for. And a separate method makes sense because it should stay that way and not be suddenly switched to HD a design change in bitcoinj. 

The other constructor (fromRandomSeed) is for starting with HD right away, something that was apparently not possible in case of a fresh wallet. You'd need to go through upgrading, which I expect can have weird side effects in apps. I'd prefer to clearly start with an HD wallet.