Enable transparent IAM backgrounds

We need to set our webview and cardview's background colors to
transparent for this. We can safely always do this because the HTML
for the IAM will already set a background color.

Author: emawby

OneSignalSDK/onesignal/src/main/java/com/onesignal/InAppMessageView.java

@@ -92,6 +92,7 @@ interface InAppMessageViewListener {
 
     void setWebView(WebView webView) {
         this.webView = webView;
+        this.webView.setBackgroundColor(Color.TRANSPARENT);
     }
 
     void setMessageController(InAppMessageViewListener messageController) {
@@ -372,6 +373,7 @@ private CardView createCardView(Context context) {
         cardView.setClipChildren(false);
         cardView.setClipToPadding(false);
         cardView.setPreventCornerOverlap(false);
+        cardView.setBackgroundColor(Color.TRANSPARENT);
 
         return cardView;
     }