Declare the headers of the ndk_library

This creates a dependency edge between the ndk_library and its headers,
which should be a no-op in regular Soong builds. This dependency edge
will be used in the Multi-tree project to export the relevant .h files into a well
known location

Test: m nothing
Bug: 239044713
Change-Id: I353794e005f69dc349ad15db47c8881dd506dff2