Call TemporaryFileProvider.cleanup() in Handler

This *might* fix Samsung problems. Let's see if it reduces crash reports.

See also:
https://stackoverflow.com/questions/41474623/sporadic-illegalargumentexception-unknown-url-content

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/provider/TemporaryFileProvider.java

@@ -111,6 +111,7 @@ public class TemporaryFileProvider extends ContentProvider {
     }
 
     public static int cleanUp(Context context) {
+        Timber.d("Cleaning up temporary files…");
         return context.getContentResolver().delete(
                 CONTENT_URI,
                 TemporaryFileColumns.COLUMN_TIME + "< ?",