This CL exposes startTethering and stopTethering functions which also
encapsulate all provisioning check logic. Right now, only silent checks
are implemented, but UI checks will come in a follow-up CL. GTS tests
and Settings changes are under the same topic ID.
BUG: 26247383
Change-Id: I65f61d899594cb3f9035d8496366af17a57a090f