Fix Bluetooth tethering

Fix Bluetooth tethering

Ignore linkstate down events for bt-pan interface which cause its remove.
Interface should only be removed when we receive the interface remove event.

At connect init to bluetooth PAN service, bridge pan device adds tetherable
bt-pan interface as set in bluetooth regexs. Before bt-pan interface gets
tethered, multiple linkstate events are received by linkstate change notifier.
Receiving multiple time of linkstate down may cause remove of tetherable
interface (bt-pan), before we call tether method.
Finally when tethering method is called and bt-pan tether interface
is removed, error is returned because of not available tetherable interfaces.

Bug found during bluetooth PTS PAN testing. It affects multiple test cases.

Change-Id: Ifb0d16657ca238aa62064fee0fc4ee974a2518ad