I have provided screenshots of the config.xml path and the configuration where you will find the package id. `
If you are using cordova from command-line, the default activity class for Cordova android application is CordovaApp.
So for in this case your configuration in Facebook developer should be like this.
Package Name: com.phonegap.helloworld
Default Activity Class Name: com.phonegap.helloworld.CordovaApp
You need to update the package name as per the entry you have in the config.xml

If you are building your app using the Adobe Phonegap Build Service,in that case the default activity name is generated by CamelCase notation of your application name.
So Default Activity Test
becomes DefaultActivityTest
So your configuration in Facebook developer should look like this.
Package Name: com.phonegap.randomPackage
Default Activity Class Name: com.phonegap.randomPackage.DefaultActivityTest

You can decompile your apk from the site below:
http://www.decompileandroid.com.
Choose the static version(the flash version was not working) and after uploading,you can see the decompiled AndroidManifest.xml file.
This is what I got after decompiling my application apk.
http://www.decompileandroid.com/complete/182864?key=ecb3cec5a9b1418afe5d67f7151b518f
