Android: Why ActivityManagerNative is not resolvable

I am trying to trace through Android SDK code in my application. I get inside android.app.Instrumentation, there is a class called ActivitymanagerNative, that is not resolved. I can't access its code. However it exists inside my SDK paths

$ find ~/android-sdk-macosx/ -name ActivityManagerNative.java
/Users/...../android-sdk-macosx//sources/android-14/android/app/ActivityManagerNative.java
/Users/...../android-sdk-macosx//sources/android-16/android/app/ActivityManagerNative.java
/Users/...../android-sdk-macosx//sources/android-17/android/app/ActivityManagerNative.java

It is in the same package, android.app.

I have attached an image that shows it is not resolved.

Thanks!