The platform as a whole is not "an app," but it is certainly possible
to set up an Eclipse project that covers basically all of the Java
portions of the source base. This is done routinely by Google
engineers working on Android.
Create a new Eclipse project "from existing sources," and point it at
the root of the platform source tree that you have checked out with
repo. Once it's in place, the repository contains the file
development/ide/eclipse/.classpath. Copy that to the root of
Eclipse's project directory, replacing whatever .classpath file was
automatically generated, and 'Refresh' the project within Eclipse.
You should be good to go at this point.
(Or steps roughly to that effect; it's been a while since I had to do
this from scratch. Good luck!)
--
chris tate
Android framework engineer