Add filtration logic to Assisted Dialing Settings.

The filtration logic will only show countries
in the country picker if the feature is enabled
for those countries.

This will not cover the regressive case where
a user has already selected a default option,
but then the option is later removed. This
behavior is currently undefined.

Bug: 69274259
Test: unit tests
PiperOrigin-RevId: 177881199
Change-Id: I113f6d25249c42c1bd98383fd6ee0bc6a8c0e615