Nullify stl and sanitize of llndk_headers

llndk_headers can't depend on other non-llndk cc modules such as libc++,
just like llndk_library. This nullifies stl and sanitize to prevent such
dependencies.

Bug: 160639583
Test: m nothing, see dependencies of llndk_headers
Change-Id: I3c6830d6b46754981c4ae10a7e7e013ec849efaf