The file navigation is as simple as using the .net 2.0 file browsing functions.
http://msdn.microsoft.com/en-us/library/system.io%28v=vs.80%29.aspx
However you will have a lot of headaches trying to get Unity to play ball with Android's Intent broadcasters/receivers and will require a lot of fiddling with Java to launch the other Activity.
http://developer.android.com/training/basics/firstapp/starting-activity.html