Reapply "Use partition intead of "root" to assemble filesystem artifacts"

This reverts commit 135904463f96f81bd7ef7696569b02ebb61c8195.

The revert will be applied alongside a change in build/make that ensures
that the correct intermediates dir for aosp_shared_system_image is
packged into target_files.zip

Test: Previously failing OTA test now passes https://android-build.corp.google.com/builds/abtd/run/L04100030008317770
Change-Id: Ibdf3ae42348ec5840dcada8ce3182f87d4619cac
diff --git a/filesystem/filesystem.go b/filesystem/filesystem.go
index 1d32b8f..b7a3adc 100644
--- a/filesystem/filesystem.go
+++ b/filesystem/filesystem.go
@@ -465,7 +465,7 @@
 }
 
 func (f *filesystem) appendToEntry(ctx android.ModuleContext, installedFile android.Path) {
-	partitionBaseDir := android.PathForModuleOut(ctx, "root", proptools.String(f.properties.Base_dir)).String() + "/"
+	partitionBaseDir := android.PathForModuleOut(ctx, f.rootDirString(), proptools.String(f.properties.Base_dir)).String() + "/"
 
 	relPath, inTargetPartition := strings.CutPrefix(installedFile.String(), partitionBaseDir)
 	if inTargetPartition {
@@ -555,8 +555,12 @@
 	builder.Command().Textf("cp -prf %s/* %s", rebasedDir, installPath)
 }
 
+func (f *filesystem) rootDirString() string {
+	return f.partitionName()
+}
+
 func (f *filesystem) buildImageUsingBuildImage(ctx android.ModuleContext) android.Path {
-	rootDir := android.PathForModuleOut(ctx, "root").OutputPath
+	rootDir := android.PathForModuleOut(ctx, f.rootDirString()).OutputPath
 	rebasedDir := rootDir
 	if f.properties.Base_dir != nil {
 		rebasedDir = rootDir.Join(ctx, *f.properties.Base_dir)
@@ -783,7 +787,7 @@
 		ctx.PropertyErrorf("include_make_built_files", "include_make_built_files is not supported for compressed cpio image.")
 	}
 
-	rootDir := android.PathForModuleOut(ctx, "root").OutputPath
+	rootDir := android.PathForModuleOut(ctx, f.rootDirString()).OutputPath
 	rebasedDir := rootDir
 	if f.properties.Base_dir != nil {
 		rebasedDir = rootDir.Join(ctx, *f.properties.Base_dir)