QtBearer networkmanager make sure to set flag Active

Also, no need to create objects to get properties, when the properties
can be had for free.

Make plugin more robust to network-manager or ofono crashes

Change-Id: Ibadb46bd51aa27f130f8d245e8c50aa7bff5f9c8
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Reviewed-by: Timo Jyrinki <timo.jyrinki@canonical.com>