Sony is working on releasing some functional versions of Android Lollipop based on open-source code. They’ve managed to complete some AOSP builds for its flagships, including Xperia Z1, Z2 and Z3.
The builds are not destined to end users, because they do not include reliability or functionality. Moreover, they do not include standard apps or even Google Play Store, which most people believe are stock Android features. The builds are made public mainly as a starting point for custom ROMs at the moment.
Sony has become more open minded towards bootloaders and mods, which means Sony products will be easier to use in the aftermarket development stage than other devices.
The updated binaries for the Xperia devices were not posted yet, which means users will not be able to compile versions of AOKP running on Z1, Z2 or Z3. They will be posted soon enough, as Sony officially posted, even if Sony is not going that far as to release customer-ready stock Android builds just yet.
Source: Sony Mobile Blog