Since config.py contains passwords in it, it should be protected as much as
possible. At the very least, the file permissions should be 0600 to
restrict access to user that actually edits and runs 'fdroid' commands.
This tries to auto-detect the SDK and NDK. If it can't find the SDK, it
prompts the user to type in the path. It also tries to find the most
recent version of the build-tools, and should also support the old
unversioned layout.