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/prebuilt_build_tool.go b/android/prebuilt_build_tool.go
index 17b3230..7773bf8 100644
--- a/android/prebuilt_build_tool.go
+++ b/android/prebuilt_build_tool.go
@@ -84,13 +84,12 @@
 	t.toolPath = OptionalPathForPath(installedPath)
 }
 
-func (t *prebuiltBuildTool) MakeVars(ctx MakeVarsModuleContext) {
-	if makeVar := String(t.properties.Export_to_make_var); makeVar != "" {
-		if t.Target().Os != ctx.Config().BuildOS {
-			return
-		}
-		ctx.StrictRaw(makeVar, t.toolPath.String())
+func (t *prebuiltBuildTool) MakeVars(ctx MakeVarsModuleContext) []ModuleMakeVarsValue {
+	if makeVar := String(t.properties.Export_to_make_var); makeVar != "" &&
+		t.Target().Os == ctx.Config().BuildOS {
+		return []ModuleMakeVarsValue{{makeVar, t.toolPath.String()}}
 	}
+	return nil
 }
 
 func (t *prebuiltBuildTool) HostToolPath() OptionalPath {