Fix tethering summary when airplane mode is off

When airplane mode is off, fall back to default summary
(tether off summary). It should be fine because once tether
state get updated again, it will go through original listener
to update the summary.

Change-Id: Iba9b56f452e72365ea964d841ee156a2625c0ae1
Fixes: 79721162
Test: RunSettingsRoboTests