Convert the following singletons to use ModuleProxy:

buildTargetSingleton
exportedJavaDeclarationsLibrarySingleton
javaFuzzPackager
kytheExtractAllSingleton
kytheExtractJavaSingleton
kytheExtractRustSingleton
lintSingleton
logtagsSingleton
makeVarsSingleton
ndkAbiDiffSingleton
ndkAbiDumpSingleton
phonySingleton
platformCompatConfigSingleton
sdkSingleton

Bug: 377723687
Test: Unit tests and compare the ninja and mk files generated.
Change-Id: I625ac8a209ca93755b2ba232202cfb44ecefde0a
diff --git a/android/filegroup.go b/android/filegroup.go
index 47102b9..4fad52a 100644
--- a/android/filegroup.go
+++ b/android/filegroup.go
@@ -131,10 +131,11 @@
 	return append(Paths{}, fg.srcs...)
 }
 
-func (fg *fileGroup) MakeVars(ctx MakeVarsModuleContext) {
+func (fg *fileGroup) MakeVars(_ MakeVarsModuleContext) []ModuleMakeVarsValue {
 	if makeVar := String(fg.properties.Export_to_make_var); makeVar != "" {
-		ctx.StrictRaw(makeVar, strings.Join(fg.srcs.Strings(), " "))
+		return []ModuleMakeVarsValue{{makeVar, strings.Join(fg.srcs.Strings(), " ")}}
 	}
+	return nil
 }
 
 // Defaults