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_test.go b/filesystem/filesystem_test.go
index 0ed3870..2dcb23d 100644
--- a/filesystem/filesystem_test.go
+++ b/filesystem/filesystem_test.go
@@ -181,7 +181,7 @@
 	`)
 
 	module := result.ModuleForTests("myfilesystem", "android_common")
-	output := module.Output("out/soong/.intermediates/myfilesystem/android_common/root/system/etc/linker.config.pb")
+	output := module.Output("out/soong/.intermediates/myfilesystem/android_common/myfilesystem/system/etc/linker.config.pb")
 
 	fullCommand := output.RuleParams.Command
 	startIndex := strings.Index(fullCommand, "conv_linker_config")