找了半天没找到，还是用google好啊！修改如下，打开如下的build.gradle，修改compileSdkVersion 为你下载的源码版本号。

Open your sdk manager from toolbar menus and download the "Sources for Android SDK" for the API level defined in your build.gradle file like

compileSdkVersion 19 // compile with API level 19

Nothing else needs to be done.

Note : Sources for SDK is available only for API levels 14 and above.

参考了stackoverflow。

AndroidStudio添加Android源码