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