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
Merged-In: Id089375a5699ac1157fc5f445876fee558d7bc7e
Merged-In: I1fc87f9c2a31f7fd16889275e6ba197b2b48a264
Merged-In: Ia6276ada690aa3e4870bb83dc4bf3ddcddc35e7b
(cherry picked from commit 6ee73daa)

Change-Id: Ic51123e173cc29cfcb52210707f2a0f8d2ab2e3b