Fix IsolatedStorageFile.GetUserStoreForApplication() for MOBILE.
IsolatedStorageFile.GetUserStoreForApplication() would previously throw an
IsolatedStorageException because no ApplicationIdentity was provided by the
AppDomain.
We would like to provide this method in a workable fashion to facilitate code
portability with Windows Phone 7, which exposes only this method for saving
files to stable storage.
Thus, skip the ApplicationIdentity and code identity checks for MOBILE
(MonoDroid, MonoTouch) so that IsolatedStorage Just Works.