Add .cargo/config to .gitignore, move content to ports/android

This allows using .cargo/config at the root of the repository
for local path override, without risk of accidentally committing it.
auto merge of #4099 : SimonSapin/servo/cargo-config, r=mbrubeck

This allows using `.cargo/config` at the root of the repository for local path override, without risk of accidentally committing it.

However I don’t have the environment to test the Android build. Would this break it?