I checked the page
http://luke.dashjr.org/programs/bitcoin/files/charts/security.html which tracks the different clients in existence and I found strange that BitcoinJ (/BitCoinJ:xxx) is not listed (since it's one of the best clear re-implementations of the protocol). I suppose that wallet applications that use Bitcoinj change the id.
Android Wallets may not be much time online, so that may be the reason. There are several that I know are based on Bitcoinj such as "Bitcoin wallet" and "Blockchain.info".
Any idea?
As far as I can tell, that's because BitcoinJ is not a full node - ie, it doesn't implement the full protocol.