Build error when using DroidPlugin as a module

Information:Gradle tasks [assembleDebug]
/home/zelongg/Code/LearnAndroid/DroidPlugin/src/com/morgoo/droidplugin/hook/proxy/IActivityManagerHook.java
Error:(105, 74) error: cannot find symbol class Singleton
Error:Execution failed for task ':DroidPlugin:compileDebugJavaWithJavac'.
> Compilation failed; see the compiler error output for details.
Information:BUILD FAILED
Information:Total time: 47.352 secs
Information:2 errors
Information:0 warnings
Information:See complete output in `console```

FieldUtils.writeStaticField(cls, "gDefault", new android.util.Singleton() {

https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/util/Singleton.java
new android.util.Singleton has a @Hide, how can we find the symbol class Singletion?