Add a TetherProvisioningActivity for ui-based provisioning.

This activity acts as a proxy between Tethering.java and the oem
provisioning app on the device. It starts the provisioning app and
relays the response back to a ResultReceiver callback.

BUG:26247383
Change-Id: I35fa6cf45f8ebacceac3e3500b89c798a650caf0