Bluetooth test interfaces: preparing for open-source

- Adding license file and headers
- Adding contributing guidelines
- Changing devsite relative links to repo relative links
  Requires running a script after copybara in Google3 to
  generates them
- Removing symbolic link to README, using a copybara workflow
  instead to rename it (since README files cannot be used in
  devsite)

Change-Id: I0bb2ef9ca72784b1981f846bf9bf4dd276f64dfe