" For a while now, Google has been developing most parts of Android behind closed doors in its “internal branches,” with the “AOSP branch” only having certain other aspects of Android’s framework (including Bluetooth, kernel, and some other core components). As such, it’s been quite a while since the current state of AOSP is at the same level as Google’s internal builds, leaving developers and others to wait on Google to make a public release to get all of the new changes.
With this change to move everything to its internal, private development branch, Google isn’t changing the speed at which these new builds arrive. Rather, this will potentially streamline the process and prevent conflicts when merging the branch"