I wanted to know what it takes to build the App-icons or the graphics that we see in an Android application, from the point of view of the tools required for that and/or the number of people that are required to build it, or, for building a graphic game...
Most of the designers we work with use Photoshop as their main tool to create UI layouts and cut graphic assets, but this is for traditional app development. Game development studios may have entire art departments working on the assets for a game, depending on how complex is it. The number of designers that would be needed to complete your project depends 100% on the time you have to complete the work and how complex the application's UI is. I don't have many tips in regards to game development, there is another book written by an Apress author that deals with the issues of developing games specifically called Beginning Android Games that you may want to investigate: http://www.apress.com/9781430246770
However, for the more traditional pieces, like icons and backgrounds, there are tools on the web and in the Android SDK. The best site I'm aware of is the Android Asset Studio, which was created and is maintained by a developer on the Android team at Google. You can use this tool to create application icons and other icon assets you may need, and they will be scaled for all device densities. Last time I checked, much of this functionality has been added to the ADT plugin for Eclipse as well when you create a new project.
There is also a tool in the SDK called draw9patch that can assist you with creating a 9-patch image from a static PNG. 9-patch images are small assets designed to stretch to fill the space required on any given device. You can find out more about them from the Android developer documentation, as we try to cover the basics of creating flexible graphic assets in our book as well.