Core: fix BT on Linux, workaround Qt bug on 5.12.0

After upgrading to Qt 5.12.0, download over BT from a DC did not work
any more. On the console the message "Connecting to port is not
supported (Uuid required)". Linus noticed earlier that we do rather
strange processing in this part of the code related to selecting port 1
or port 5. This all seems not needed (any more), but broader testing is
advised. This being stripped from the code, the mentioned error from Qt
persisted. That is strange in itself, as we did not reference port
numbers any more.

Step 2 in this commit is actually using an uuid to the call to
connectToService. Choosing an uuid seems relatively straightforward as
we can use the same one we already use for Android. That is the default
BT RFCOMM Serial Port Profile uuid. Interestingly, when changing to this
uuid we run immediately in a Qt runtime error telling us "QDBusPendingReply:
type ManagedObjectList is not registered with QtDBus.". For these 2
unexpected Qt messages, QTBUG-72742 was made. Studying the Qt source
code at this point reveals a possible workaround. Simply create a local
QBluetoothLocalDevice object, which, behind the scenes registers the Qt
internal ManagedObjectList with QtDBus.

In the meantime, Qt agrees that QTBUG-72742 is valid, and that a fix is
to be expected in a future version. At that point in time, the
declaration of the QBluetoothLocalDevice can be deleted again.

In the end, interfacing over BT works again.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>

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