InAppBrowser should use separate OverrideUserAgent from CDVViewController

Currently, there is an OverrideUserAgent setting that will affect both the core webview (i.e., that runs the main web app) as well as the inappbrowser plugin. It would be great to be able to separate into two setting, i.e., have an InAppBrowserOverrideUserAgent. That's because I'd like my own app to know the true useragent, while sending a different useragent to other web sites.