Make bluetooth payload upload work & asynchronous
Summary: Sharing files to the Android bluetooth backend now works. The code is asynchronous now too.
Test Plan: I tested several files, and they all get transferred correctly. The socket gets destroyed correctly too.
Reviewers: KDE Connect, apol
Reviewed By: KDE Connect, apol
Subscribers: apol, KDE Connect
Tags: KDE Connect
Differential Revision: https://phabricator.kde.org/D12264