Suppress WiFi Assistant card until future platform updates.

This replaces the previous behavior of an increasing timeout. When the
user dismisses the WiFi Assistant card, either by opting out or
successfully configuring the assistant, the card is suppressed until
the next major platform upgrade (assuming the assistant is not enabled
at the time of the upgrade).

Bug: 16902858
Change-Id: I5373a7a743f21777b7b75a620b5a4e9004e57466