Return false when tetherOffloadRuleAdd doesn't really add items

This is needed for the follow-up CLs to check whether an item was
added when calling tetherOffloadRuleAdd. This is safe since no one
is actually checking the return value of this method.

Test: atest TetheringTests
Bug: 354619988
Change-Id: I28bafe3145bd2f62f9a7db9d0a300d2c7a148040
2 files changed