Rather than use the crufty config.xml list of upstream transport types,
use ConnectivityService's notion of the default network for the upstream.
In cases where a DUN network is required and the default network is
currently a mobile network, look for a DUN network (code in Tethering
is currently responsible for requesting one).
Test: as follows
- built, flashed, booted
- runtest frameworks-net
- tethered via mobile, joined captive portal network, maintained
laptop access via mobile until captive passed (then used wifi)
- disabled client mode wifi, disabled mobile data, plugged in
ethernet adapter, observed connectivity via ethernet
Bug: 32163131
Bug: 62648872
Bug: 63282480
Bug: 109786760
Bug: 110118584
Bug: 110260419
Change-Id: I925b75994e31df8046f3ef9916a2457b4210485e