create .buildinfo to document essential details for reproducible builds
Most of the build setup is already present in the metadata file, but there are inevitably some details that cannot be specified in the metadata file that will affect the build. In order to have reproducible builds, we need a mechanism to document those details for every build. This is generally known as a "buildinfo" file, and Debian, Maven, etc. already provide them. I think the F-Droid buildinfo files will be quite a bit simpler than Debian, and quite close to Maven. So I propose we adopt the Maven/JVM .buildinfo format.
Here are somethings that need to be included:
- actual commit ID of the source repo used in the build
- exact version of JDK
- versions from source.properties from Android SDK components, in case Google does anything stupid again