Do not export private headers with llndk library

A header file in private directory are not supposed to export with
llndk library. To avoid this, move the private directory to a new
include-private directory and export it for framework-only library.

Bug: 69235725
Test: build and boot any device.
Change-Id: I9f020411350cf5b5f1db01ef9add76d8c3b2c395