I have this grade file:
//ext.support_library_version = '24.0.0'
Sources for Android N are not yet published and not available for installation in the SDK manager. When the source package becomes available, you need to install it separately, as with any previous SDK version.
Update 2016-08-24: The Android API 24 sources are now available in the SDK manager.
After downloading you might need this trick to refresh the SDK installation.