Try to repair it booted from a live system: Boot from your Ubuntu install CD/USB drive.
Run and to identify your partitions.
Mount the root partition of the installed ubuntu system, probably /dev/sda1, /dev/sda5 or /dev/sda6. Use the file browser to mount/umount. Let's say your system is mounted on /media/ubuntu (the name may vary depending of if you use labels).
and edit the file with
sudo cp /media/ubuntu/etc/environment /media/ubuntu/etc/environment.old
But if that file is good, and your path is still bad, it is changed somewhere. Check also the locations suggested by Lars Noodén, where ~ should translate to something like
sudo nano /media/ubuntu/etc/environment
when running the live system.
You might also try to search for PATH= in the /etc directory
and for android-sdk-linux
sudo grep -r "PATH=" /media/ubuntu/etc/*
which might help you find where your path is reset.
sudo grep -r "android-sdk-linux" /media/ubuntu/etc/*