[AC-6542] Admob module iOS version doesn't honor adBackgroundColor

The adBackgroundColor property has no effect in ti.admob iOS version. Banner Ads always appear with black background regardless of whatever color you set using this property. *Steps to reproduce:* 1. Create new Titanium app. 2. Download ti.admob module. 3. Copy the example app.js code in index.js and change adBackgroundColor to red (or whatever else) 4. Run on iOS simulator or device. *Expected Result:* Ad background should be appear as passed while creating ad view. *Actual Result:* Empty background space on ad view is always black.