If you're using Tier 1 (BASIC) then you have to now use the InAppPurchaseSetKeys command.
If you're using Tier 2 then it is in
C:\Program Files (x86)\The Game Creators\AGK2.0.20\Tier 2\apps\interpreter_android_google\AGKPlayer2\src\main\java\com\thegamecreators\agk_player\AGKHelper.java
Looks like it is around line 1957
A search for base64EncodedPublicKey should find it for you.
However, this line is overwritten by iapSetKeyData() which is likely the method that is called by InAppPurchasesSetKeys.
So if you want to modify it in the java file you might be best off by changing it in both places. But likely it is safest to use InAppPurchasesSetKeys.