What is it already?

Android Rebuilds provides freely-licensed builds of Android development tools written by somebody else.

New builds

SDK 10 (API 29) and NDK 20 rebuilds are now available, as unattended build scripts as well as binaries you shan't trust.

sdkmanager integration will be complete when we figure out how to give our repo precedence over somebody else's.

Evolution of the situation

SDK build remains monolithic and growing (40GB .git, 7h multi-core build, 200GB build space).

But there are fewer build issues, thanks to newer "prebuilts" deps straight in Git, now including OpenJDK.
I expect we'll soon chroot in Git before build.

Also for the first time ever I could complete a NDK windows build.

Licensing

Official binaries are still click-wrapped with a proprietary license.

It was discovered that such a license is also covering past versions of android.jar & al. hidden in a prebuilts directory and somehow necessary to the builds.
Archeological work already successfully started to rebuild SDKs from the start of the decade.

Fanbase

Android Rebuilds is showcased in ungoogled-chromium-android, a lightweight approach to removing Google web service dependency.

F-Droid mirror

After some back and forth, the F-Droid mirror is stable and limited to the experimental sdkmanager repository.
F-Droid showed high dedication to implementing upload restrictions and establishing procedures.
I have great hope that they will soon show the same level of dedication dropping non-free licenses and freeing their build server.