Make WifiTether listen to datasaver

Wifi Tethering is already turned off when appropriate. However
if you re-enable it you still can't toggle the tether switchbar.
This CL makes it so that if you disable data saver the toggle
becomes enabled again.

Test: robotests
Bug: 77860936
Change-Id: I03fa74c21674678a90cde7bd20fa68167f02ae3c