I tried in the play store, but I didn't get the app. So do we have this app there, and also if we don't have then, can we have this in future. I think it will promote StackExchange.
Not Official but there are various unofficial apps, one of them is Stack Remote (No product placement intended and its not even preferred by me).
Update: Finally we have an Android App.
We are one of many many Stack Exchange sites.
As @ChristianRau says - this might be better asked on meta.stackoverflow.com - which is the meta site for the entire StackExchange network. Check there next, there may be already questions on here about this.
That said, I do not believe there is an official mobile client for any platform, iOS, Android or Blackberry. I have however come across third party apps for Stackoverflow.