Fix IAM Preview Notification being handle as normal Notification

[Jeasmine]

### Bug
IAM Notification was being handled as a normal notification; this means that notifications callbacks from the user were being called and receive receipt request was also made for this notification.

### Solution
Currently, we have IAM handling, but only if the app is in the foreground. Add Notification display to the same inAppMessagePreviewHandled method. This helped cleaning some code, checking if IAM is preview inside common notification display flow was removed.

Review commit by commit to seeing the process of fixing

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This change is 

[jkasten2]

👍 Nice overall cleanup. A few optional comments.

Reviewed 7 of 7 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @emawby and @Jeasmine)

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OneSignalSDK/onesignal/src/main/java/com/onesignal/OSInAppMessagePreviewHandler.java, line 15 at r1 (raw file):

import static com.onesignal.NotificationBundleProcessor.PUSH_ADDITIONAL_DATA_KEY;

class OSInAppMessagePreviewHandler {

Since this is a new class what do you think about using Kotlin instead?

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OneSignalSDK/onesignal/src/main/java/com/onesignal/OSInAppMessagePreviewHandler.java, line 21 at r1 (raw file):

        // Show In-App message preview it is in the payload & the app is in focus
        String previewUUID = inAppPreviewPushUUID(pushPayloadJson);
        if (previewUUID != null) {

Would be a bit easier to read if this was an early return instead.

 if (previewUUID == null) {
    return false;
}

[emawby]

Reviewed 7 of 7 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @Jeasmine and @nan-li)

[jkasten2]

Reviewed 2 of 2 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @Jeasmine and @nan-li)