Flatten the transitions to tethering and hotspot.

The previous version could trigger a Log.wtf() if an error occured within
ServingState.enter() that caused a transition while a tranistion was
already in flight.

Test: as follows
    - build
    - flashed
    - booted
    - "runtest frameworks-net" passes
Bug: 31466854

Change-Id: I66a47caf22e39ab4ad7f4ddd60940e9fe1580729