Google has just announced the release of Android 5.1, still called Lollipop, which mostly improves performance and stability, but also adds a few features.
- Multiple SIM slots support – Allows to use 2 or 3 SIM cards in your phone to better manage your mobile costs.
- Device Protection – Your lost or stolen device will remain locked until you sign in with your Google account – even if someone resets your device to factory settings. Google says it will be available on most Android phones and tablets shipped with Android 5.1, implying it might not be supported on all.
- High Definition voice calling
- Join Wi-Fi networks and control of your paired Bluetooth devices directly from Quick Settings.
Factory images can already be downloaded for Nexus 5, Nexus 10, and Nexus 7 Wi-Fi (nasaki), but Android 5.1 firmware should eventually become available for other recent Nexus devices too.
You should be able to get the source code from AOSP with the commands:
repo init -u https://android.googlesource.com/platform/manifest -b android-5.1.0_r1