Merge changes from topic "header_libs_requirement"

* changes:
  Add libstagefright_mp3dec_headers to allowed apex deps
  Require libraries in header_libs to be cc_library_header

cc/linkable.go

@@ -130,6 +130,13 @@ type StaticLibraryInfo struct {
 
 var StaticLibraryInfoProvider = blueprint.NewProvider(StaticLibraryInfo{})
 
+// HeaderLibraryInfo is a marker provider that identifies a module as a header library.
+type HeaderLibraryInfo struct {
+}
+
+// HeaderLibraryInfoProvider is a marker provider that identifies a module as a header library.
+var HeaderLibraryInfoProvider = blueprint.NewProvider(HeaderLibraryInfo{})
+
 // FlagExporterInfo is a provider to propagate transitive library information
 // pertaining to exported include paths and flags.
 type FlagExporterInfo struct {