Calendar: Fix check for things to import

* hasThingsToImport() doesnt require a context
* Only should return true if there are actually files, not folders
--> use listFiles() instead if list()

Change-Id: Id9d5f7e40f4ee644dbb055dcc86377c2e71dd887
(cherry picked from commit 573612e897cfe55775334d9d26d8a552c58467f2)
(cherry picked from commit f42d5a7)