Duplicated onInterstitialAdShown callbacks in Flutter plugin

[AminPourabazari]

When using the 'adivery' Flutter plugin (v4.8.x), I observe that _onInterstitialShown callback is triggered multiple times (8+) for a single interstitial ad display, even though initialize() is only called once.

-- I traced the issue to the native plugin implementation in AdiveryPlugin.java:
It calls Adivery.addGlobalListener(listener) on each initialize() call, without checking isInitialized. As a result, multiple listeners accumulate, causing duplicate callback invocations.

Suggested fix:

case "initialize":
    if (!isInitialized) {
        Adivery.configure(...);
        Adivery.addGlobalListener(listener);
        isInitialized = true;
    }
    break;

Also in onDetachedFromEngine, add:

if (isInitialized) {
    Adivery.removeGlobalListener(listener);
    isInitialized = false;
}