config: Use tether automatic upstream selection

(config_tether_upstream_automatic=true)

Test: as follows
    - built, flashed, booted
Bug: 32163131
Bug: 62648872
Bug: 63282480
Bug: 110118584
Bug: 110260419

Change-Id: I604cda53d6d37a33de3e3ccc65c0da781d21278b