Add a QtAndroid::startActivity that takes a QAndroidIntent

The function simply calls the other startActivity with
QAndroidIntent::handle, anyone could do that from the user code, but i
think it makes sense the library to allow you doing this directly given
you're supposed to pass an intent and we have a class that represents an
intent

Change-Id: I673cb410b07ed4fd1408a971696a3b5ae3dfd063
Reviewed-by: BogDan Vatra <bogdan@kdab.com>