Remove transient StartingState from TetherInterfaceSM

This state immediately switched into the TetheredState.
We can safely remove it at only the expense of a misleading state
broadcast.

Bug: 28798823
Test: runtest frameworks-services -c
    com.android.server.connectivity.tethering.TetherInterfaceSMTest

Change-Id: I295d830dbdb11717d6e05161d97787ffe871beb3