android_system_image only packages "system" items

android_system_image filers packaging items installed outside "system"
partition.

Some packaging items install related items to different partitions but
putting them altogether to android_system_image doesn't make sense.
(android_system_image is suppposed to be "system" partition)

To be specific, this filters out "apex" partition items.  "apex"
partition is used by APEX installation to install APEX contents to paths
similar to activated paths on device so that symbol lookup works well
with APEX contents.

Bug: 225121718
Test: atest MicrodroidHostTestCases
Test: debugfs <intermediate>/microdroid.img -R 'ls system'
  shows no "com.android.runtime"
Change-Id: Ibc3d85ead2fda99e231132ce8ab9ccf1cc9317b7

filesystem/system_image.go

@@ -37,6 +37,7 @@ func systemImageFactory() android.Module {
 	module := &systemImage{}
 	module.AddProperties(&module.properties)
 	module.filesystem.buildExtraFiles = module.buildExtraFiles
+	module.filesystem.filterPackagingSpecs = module.filterPackagingSpecs
 	initFilesystemModule(&module.filesystem)
 	return module
 }
@@ -53,7 +54,7 @@ func (s *systemImage) buildLinkerConfigFile(ctx android.ModuleContext, root andr
 
 	// we need "Module"s for packaging items
 	var otherModules []android.Module
-	deps := s.GatherPackagingSpecs(ctx)
+	deps := s.gatherFilteredPackagingSpecs(ctx)
 	ctx.WalkDeps(func(child, parent android.Module) bool {
 		for _, ps := range child.PackagingSpecs() {
 			if _, ok := deps[ps.RelPathInPackage()]; ok {
@@ -68,3 +69,14 @@ func (s *systemImage) buildLinkerConfigFile(ctx android.ModuleContext, root andr
 	builder.Build("conv_linker_config", "Generate linker config protobuf "+output.String())
 	return output
 }
+
+// Filter the result of GatherPackagingSpecs to discard items targeting outside "system" partition.
+// Note that "apex" module installs its contents to "apex"(fake partition) as well
+// for symbol lookup by imitating "activated" paths.
+func (s *systemImage) filterPackagingSpecs(specs map[string]android.PackagingSpec) {
+	for k, ps := range specs {
+		if ps.Partition() != "system" {
+			delete(specs, k)
+		}
+	}
+}