Tldr:

To balance AOSP’s open nature with its product development strategy, Google maintains two primary Android branches: the public AOSP branch and its internal development branch. The AOSP branch is accessible to anyone, while Google’s internal branch is restricted to companies with a Google Mobile Services (GMS)licensing agreement.

Beginning next week, all Android development will occur within Google’s internal branches, and the source code for changes will only be released when Google publishes a new branch containing those changes. As this is already the practice for most Android component changes, Google is simply consolidating its development efforts into a single branch.