I know this is old an old post, but I thought I’d add an answer as I’ve recently had to do this myself.
Add something like this (tailored to your project obvisouly) to your UPL file to add the entry to the android manifest: (got the information from here: Android: how to add an xml file to res/xml - UE4 AnswerHub )
And in the AndroidManifestUpdates section, add this:
<!-- Facebook network security configuration -->
<addAttribute tag="application" name="android:networkSecurityConfig" value="@xml/network_security_config"/>
Then place your network_security_config.xml file in (YourProject)/Plugins/(YourPlugin)/Source/(YourPlugin)/ and it will copy it from there during the build.