Stop unnecessary tethering reconfigurations

The TetheringConfiguraiton is retrieved at startup and on every
ACTION_CONFIGURATION_CHANGED broadcast.  Re-retrieving the config
at every upstream selection is unnecessary and fills up the logs.

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
Bug: 32163131
Bug: 63250751
Change-Id: Ia6276ada690aa3e4870bb83dc4bf3ddcddc35e7b