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According to Google "the N Developer Preview runs from 9 March 2016 until the final Android N public release to AOSP and OEMs, planned for Q3 2016.". So, for now, the sources have not been shared via AOSP.
During the preview period testing of the preview is only possible on devices for which Google provides binaries. These are listed at http://developer.android.com/preview/download.html (and Hikey is not among them).