Clear currentShareInfo when transfer is complete

Summary:
currentShareInfo needs to be nulled when a transfer is complete if not the next transfer will
have incorrect notification information and will never finish

Test Plan:
Send 2 files from desktop to android, notification is as expected (eg (File 1 of 2) followed by (File 2 of 2)
When transfer is complete send 2 files again, notification is as expected and is finished when transfer is done

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17259