Fix crash during service discovery on Android

This is caused when running a service discovery on an Android emulator.
The emulator does not have a local Bluetooth device despite the Qt code
assuming it.

Change-Id: I9c0d826d14e7494bfeb27d449f9b6f568860d917
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>