Use Android toolchain file from ECM CI artifacts

This works for all packages depending on ECM. In particular, it doesn't work for ECM itself though. We therefore expose the toolchain path as a Gitlab variable that ECM can then set to the path in its own source dir. If we have repos that cannot depend on ECM for whatever reason, setting that to the path in the image is also an option.

Remaining questions:

• does this actually work conceptually? I am just guessing from reading the Gitlab docs: https://docs.gitlab.com/ee/ci/yaml/includes.html#use-variables-with-include
• what are the correct paths to use here and for ECM itself?
• ECM needs the variable set before we integrate this here