Remove llndk_headers

Replace llndk_headers with cc_library_headers with
llndk.llndk_headers: true.

Bug: 170784825
Test: m checkbuild
Test: compare out/soong/build.ninja
Test: TestLlndkHeaders
Change-Id: I33b411cd4d474318796c2073375176b82dd8c216

rust/rust.go

@@ -262,10 +262,6 @@ func (c *Module) IsLlndkPublic() bool {
 	return false
 }
 
-func (m *Module) IsLlndkHeaders() bool {
-	return false
-}
-
 func (m *Module) IsLlndkLibrary() bool {
 	return false
 }