Introduce soong_filesystem_creator module type

The module generates soong-built filesystem modules. This module creates
the system partition submodule, and will create other partition
filesystem submodules in the future.

Note that not all properties of the system submodule is currently set;
some properties like `deps` are not currently set.

Test: m nothing --no-skip-soong-tests
Bug: 369682648
Change-Id: I671a9b6a61e5453117f604394191243c5ae047d9
diff --git a/filesystem/system_image.go b/filesystem/system_image.go
index 63cb627..57239ae 100644
--- a/filesystem/system_image.go
+++ b/filesystem/system_image.go
@@ -33,7 +33,7 @@
 // android_system_image is a specialization of android_filesystem for the 'system' partition.
 // Currently, the only difference is the inclusion of linker.config.pb file which specifies
 // the provided and the required libraries to and from APEXes.
-func systemImageFactory() android.Module {
+func SystemImageFactory() android.Module {
 	module := &systemImage{}
 	module.AddProperties(&module.properties)
 	module.filesystem.buildExtraFiles = module.buildExtraFiles
@@ -42,6 +42,10 @@
 	return module
 }
 
+func (s systemImage) FsProps() FilesystemProperties {
+	return s.filesystem.properties
+}
+
 func (s *systemImage) buildExtraFiles(ctx android.ModuleContext, root android.OutputPath) android.OutputPaths {
 	if s.filesystem.properties.Partition_type != nil {
 		ctx.PropertyErrorf("partition_type", "partition_type must be unset on an android_system_image module. It is assumed to be 'system'.")