Prefer default Internet network for upstream tethering.

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