On the website android.googlesource.com are all android platform sources available via git repositories or tarballs.
As an example if you want to get the source for the version 4.1.2
To grad all the files in a tarball you can use
Generation of standard javadocs
On windows I got some errors when trying to untar the tarball. Using linux (debian in my case) is a better choice.
After untar of the tgz change to the directory and enter a command like
find . -type f -name "*.java" | xargs /opt/jdk/jdk1.8.0_05/bin/javadoc -d /bigtmp/android-4.1.2/javadoc/all/
Maybe the path to javadoc has to be adapted to your distribution.