Make auto-upstream the default tethering behaviour

Test: as follows
    - built
    - flashed
    - booted
    - runtest --no-hidden-api-checks frameworks-net
Bug: 32163131
Bug: 62648872
Bug: 63282480
Bug: 110118584
Change-Id: I3af6594cd16ecefa3d4a44902f2138fa635fbae1