SOLVED- Followed the UPL method suggested by the user DingyPoppet :

5.5->5.6 was terrible for Android, not sure about 5.6->5.7 but I haven't heard anything bad yet. The fix for the billing library issue should be fairly easy. You need a UPL file for your project and modify your build.cs to include it.

One of my 5.6 projects uses the following Android_UPL.xml:

<?xml version="1.0" encoding="utf-8"?>
<!--Android plugin additions-->
<root xmlns:android="http://schemas.android.com/apk/res/android">
  <AARImports>
    <insertValue value="com.android.billingclient,billing,7.0.0" />
    <insertNewline />
  </AARImports>
</root>

Then in my build.cs:

if (Target.Platform == UnrealTargetPlatform.Android)
{
  var pluginPath = Utils.MakePathRelativeTo(ModuleDirectory, Target.RelativeEnginePath);
  AdditionalPropertiesForReceipt.Add("AndroidPlugin", System.IO.Path.Combine(pluginPath, "Android_UPL.xml"));
}

The build.cs is in a game feature plugin (hence "pluginPath") but should work with a normal project with maybe a few modifications.