Writing a preference activity wrapper for android project (exported from unity)

When exporting a project, an android project is generated with the following entries in its manifest file:

Code (CSharp):

1.     <application>    
2.         <meta-data
3.             android:name="com.google.android.gms.ads.APPLICATION_ID"
4.             android:value="@string/admob_app_id"/>
5.  
6.         <!-- This is to be extended with an activity that handles in app purchases -->
7.         <activity
8.             android:name="com.co.android.unityplugin.PreferencesActivity"
9.             android:exported="true"
10.             android:hardwareAccelerated="true"
11.             android:windowSoftInputMode="adjustPan" >
12.         </activity> <!-- Without explicit hardwareAccelerated="true" , preference activity scrolling was very slow -->

I need to write a custom in app purchase mechanism since unity doesn't support IAP for live wallpapers (the project is using a plugin to this end).

Right now, I'm thinking of extending "com.co.android.unityplugin.PreferencesActivity" with a custom activity that does the IAP handling and created a separate module in the exported android project but can't seem to be able to access PreferencesActiviy:

Code (CSharp):

1. error: package com.co.android.unityplugin.PreferencesActivity does not exist.

I followed the suggestions at:

https://stackoverflow.com/questions...t-class-from-another-module-in-android-studio

Code (CSharp):

1. allprojects {
2.     buildscript {
3.         repositories {
4.             google()
5.             jcenter()
6.         }
7.  
8.         dependencies {
9.             classpath 'com.android.tools.build:gradle:3.4.2'
10.             compile project(':iappreferencemodule') // <== this is the module that handles IAP and was added
11.                                                                             //        as a dependency to all android bundles
12.         }

but got:

Code (CSharp):

1. ERROR: Could not find method compile() for arguments

and suggestions such as:

https://stackoverflow.com/questions/27617687/gradle-dsl-method-not-found-compile

do not seem to be helpful in building the project. Is this the right way of extending preference activities?