If you’ve wondered why it takes so long to roll out OS updates, it’s worth a read. It’s also worth mentioning that system-level updates now take a back seat to Google-provided core app functionality from the app store, which neatly sidesteps these issues. See here for more – http://goo.gl/5p5ErZ.
”The PDK is a collection of source files and software binaries that enable device manufacturers and chipset vendors to port their drivers, develop, optimise and test against a new Android release before it is publicly released. The Android PDK includes a set of interfaces for the Android hardware abstraction layer (HAL), platform sources for integration, and a binary system image. The PDK also includes HAL and integration documentation and the Android Compatibility Test Suite.”