[SettingsLib] Add missing AccessPoint listener

- AccessPointListener was not set on AccessPoint in one of the code
  paths and as a result, the UI wasn't notified of changes to the
  AccessPoint.

Test: Manually tested and verified that notification changes were
delivered to the UI. Also added unit test.
bug:31945554

Change-Id: I89affada5dead4edc4218f424eec6b7f4a4e26a7