mpris: Fix lastUsed metadata
Commit 39608d44 highlighted a bug which was never triggered before: the lastUsed metadata is badly formatted. Indeed, the last_played method of Grl.Media returns a GLib.DateTime. This date needs to be formatted into a string in order to be used by MPRIS. This bug had never been encountered because of the presence of a try/except block.
Fix the issue by formatting the date into a string in ISO 8601 format.
Closes: #294 (closed)