Add a Context getter in CordovaInterface

We use a WebView instance within an android Service and attach it to an activity view hierarchy when user interaction is required.
Assuming cordova wants to support such use case a Context getter is needed in CordovaInterface for plugins that are accessing Context through getActivity() (such as the device plugin)

I originally created CB-12623 for the device plugin. After receiving comments I think it should be addressed here first.