Only start tether when not already enabled

The toggle in the tether page currently tries to start tethering if
it gets turned on. However, changing a wifi config can cause
startTether to be called and also the toggle to switch leading to
a double call to startTether. This CL makes it so that if we are
already connected to tethering we don't try to start it again.

Test: robotests
Change-Id: I91085244c5d1fcc245a6b6480e638a32bc14f0ac
Fixes: 78028548
Fixes: 78441216
Fixes: 74368363