Stop using shared DUN APN when tethering stops.

Backported from master, including a bug fix and a cdma enhancement.

Even if other people are sharing the connection (ie, carrier wants
default and tethered traffic on the same APN) stop using a carrier-
described APN when the tethering stops.

bug:5972599
Change-Id: I25e4831855e6b62c0c3ab3a6f4d4846aaee6ac50