I have provided screenshots of the config.xml path and the configuration in which you will find the package identifier. `
If you are using a command line cord, the default activity class for the Cordova android application is CordovaApp.
So, in this case, your configuration in the 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 according to the entry in the config.xml file

If you create your application using the Adobe Phonegap build service, in this case the default activity name is created as CamelCase notation of your application name. So, the Default Activity Test becomes the DefaultActivityTest
So your Facebook developer configuration 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 .
Select the static version (the flash version does not work), and after downloading it you can see the decompiled AndroidManifest.xml file.
This is what I got after decompiling the apk application.
http://www.decompileandroid.com/complete/182864?key=ecb3cec5a9b1418afe5d67f7151b518f

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