apex: file_contexts for override_apex flattened

Make-var APEX_FILE_CONTEXTS_INFOS should contain entries for
override_apex modules. Otherwise flattened override_apex modules can't
be correctly labeled.

Bug: 154031888
Test: lunch gsi_gms_arm64-userdebug
      m file_contexts.bin
      check ../file_contexts.bin_intermediates/file_contexts.concat.tmp
      if it contains entries for override_apex
Change-Id: I60fc3da1def0f5fe14cfeef32a4c2a07a65b056e

apex/builder.go

@@ -587,7 +587,7 @@ func (a *apexBundle) buildFlattenedApex(ctx android.ModuleContext) {
 	apexBundleName := a.Name()
 	a.outputFile = android.PathForModuleInstall(&factx, "apex", apexBundleName)
 
-	if a.installable() && a.GetOverriddenBy() == "" {
+	if a.installable() {
 		installPath := android.PathForModuleInstall(ctx, "apex", apexBundleName)
 		devicePath := android.InstallPathToOnDevicePath(ctx, installPath)
 		addFlattenedFileContextsInfos(ctx, apexBundleName+":"+devicePath+":"+a.fileContexts.String())