Do not create a new content fragment on orientation change

Summary:
When an orientation change occurs the fragment manager will automatically
re-create the content fragment for us.

Test Plan:
Set the content fragment to DeviceFragment and change orientation
Result: DeviceFragment is shown for the correct device and the correct navigation view menu item is checked

Set the content fragment to PairingFragment and change orientation
Result: PairingFragment is shown for the correct device and the "Pair new device" navigation view menu item is checked

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17409