Cleanup the TetheredClients API

Add comments to getters as requested in API review, and remove the
expirationTime private field that was planned to be replaced with
LinkAddress expiration.

Test: atest TetheringTests
Fixes: 150878126
Change-Id: Iecf65859cdeeaac2fa7b817b4f505c510424ac89
Merged-In: Iecf65859cdeeaac2fa7b817b4f505c510424ac89
(cherry picked from commit 594d0eae38c13e2bb03de0b3ae1f8781991c321e)
4 files changed