Add methods to distinguish TetheringRequest types

Add TetheringRequest#RequestType to distinguish the type of
TetheringRequest (i.e. from startTethering, from implicit broadcasts, or
from the legacy tether API) as well as helper methods to create these
requests. These will be used in a future CL.

Bug: 216524590
Test: build

Change-Id: Ibfda48f59e80d423c9b6ee0d244cbdab14be1bd2
6 files changed