Android Question How can I build a wallet for Bitcoin?

If you have to ask you are probably far from level you need to be to achieve something like this.

There is only one project you need to know, it's used in wallets such as Bitcoin Wallet for Android, MultiBit and GreenAddress GreenBits.
https://github.com/bitcoinj/bitcoinj

You can just compile it and use it to build your B4A app or pay someone to wrap it to you. In which case we are speaking price range between 1500 - 2500 $