Make Tethering not disconnect on every net change.

Tethering will now persist to on (until turned off) through upstream net drops,
tunnels, switches to wifi or 2G, mms sends, etc.  It will also strive to use
the most natural connection and only go to HiPri/Dun APNs when needed.

bug:2519915
bug:2501837
Change-Id: Iedf6647daad8df507d7c87d1130b204b0a0acc7b