Refactor tether logic for reuse

Some logic related to permissions needs to be used elsewhere. This
CL refactors the relevant code into a method so that it stays in
sync in both places.

Test: robotests still pass
Bug: 64092292
Change-Id: If8191698023d1da90db99b518d9f770683b5c789
Merged-In: I3b54529865e16b7e1640b0adda7f7edb9d1a41f7