Pencil is a user interface (UI) prototype visual designer for your applications. Pencil is cross platform and can run on Windows, Ubuntu/Linux, and Mac OS X.
Pencil supports stencils aka collections, many of them are free. One of them is very useful which is Android UI Stencils, see example screenshots below:
You can get Android UI stencil for Pencil from Pencil downloads page.
Note: In Ubuntu/Linux, before you can run Pencil you must first install xulrunner-1.9.2 runtime. Enter this command using Terminal :
sudo apt-get install xulrunner-1.9.2-gnome-support
Tip: To run Pencil on Ubuntu 11.04 Natty Narwhal 64-bit (which I use), if the standard "pencil" script doesn't work, you may need to create a launcher like this:
Recommendation: To develop Android mobile applications using Flash Builder and Adobe AIR, check out Developing Android Applications with Adobe AIR (Adobe Developer Library).