[SB][Wifi] Don't expose WifiRepositoryImpl.selectedUserContext.

We needed to temporarily expose selectedUserContext to easily test the
new multiuser logic in WifiRepositoryImplTest. This CL updates that test
class to reset the wifiPickerTrackerFactory and use a new context captor,
which helps us verify the context via argument capture instead of
exposing the context flow.

Bug: 371586248
Flag: TEST_ONLY
Test: atest WifiRepositoryImplTest
Change-Id: Ib89eb3c63fa57aba240f3faf0cb0e282d01b6be8