Expose a simple tethering API which includes provision checks.

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