platform_bootclasspath: aggregate hidden API flag files from fragments

Aggregates hidden API flag files from the bootclasspath_fragments which
will allow the hidden API flag files in frameworks/base/boot/hiddenapi
to be modularized and moved to the appropriate repo.

Bug: 177892522
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
      also verified that changes to the fragment provided files do
      affect the monolithic files.
Change-Id: Ifce14c9ef24c58c7ab1085475d85b61cfbfefecd

java/hiddenapi_modular.go

@@ -16,6 +16,7 @@ package java
 
 import (
 	"android/soong/android"
+	"github.com/google/blueprint"
 )
 
 // Contains support for processing hiddenAPI in a modular fashion.
@@ -172,6 +173,14 @@ type hiddenAPIFlagFileInfo struct {
 	categoryToPaths map[*hiddenAPIFlagFileCategory]android.Paths
 }
 
+func (i *hiddenAPIFlagFileInfo) append(other hiddenAPIFlagFileInfo) {
+	for _, category := range hiddenAPIFlagFileCategories {
+		i.categoryToPaths[category] = append(i.categoryToPaths[category], other.categoryToPaths[category]...)
+	}
+}
+
+var hiddenAPIFlagFileInfoProvider = blueprint.NewProvider(hiddenAPIFlagFileInfo{})
+
 // ruleToGenerateHiddenApiFlags creates a rule to create the monolithic hidden API flags from the
 // flags from all the modules, the stub flags, augmented with some additional configuration files.
 //