@cosmin @fdroidorg @strypey Debian maintains an Android SDK package which appears to be free, but it isn't complete and doesn't correspond with google's version; https://qa.debian.org/developer.php?email=android-tools-devel%40lists.alioth.debian.org

It is listed to install google's proprietary SDK in the build server instructions; https://f-droid.org/docs/Installing_the_Server_and_Repo_Tools/

Google releases SDK binaries under a proprietary license that says; "3.4 You may not use the SDK for any purpose not expressly permitted by the License Agreement. Except to the extent required by applicable third party licenses, you may not copy (except for backup purposes), modify, adapt, redistribute, decompile, reverse engineer, disassemble, or create derivative works of the SDK or any part of the SDK."; https://developer.android.com/studio/terms

This may seem odd, considering that they also release the sources to the SDK under a free license that seems mostly complete (but it seems the spyware and malware injection code is missing - for example google played services stuff, which means there are slight differences).

The SDK sources are massive and are very difficult to get to compile, but some people manage to get them to compile after many hours of work and release a free SDK, which can be used to compile Android software.

But, it appears that a lot of Android software depends on proprietary stuff in the google SDK (either accidentally or intentionally), meaning that it fails to compile without modifications under a free SDK - which is quite inconvenient.

F-droid could start a drive to get stuff compiling with a free SDK, but instead they've gone the easier route (a faster road that leads to the wrong place) and is using google's proprietary SDK.