WifiCallingSettingsForSubTest: Solve casting problem in setup

Solves casting problem in WifiCallingSettingsForSubTest that causes all
tests in this file to fail in the setup-function.

The wfc mode buttons has changed from being a ListPreference to be a
ListWithEntrySummaryPreference. The test cases have now been updated
accordingly.

Problem was introduced by the following two patches that conflict with
each other. The test cases in WifiCallingSettingsForSubTest works
correctly when each patch is tested in isolation, but failed when they
were both merged:
73ffcf49
8498436a

Bug: 129545431
Test: make RunSettingsRoboTests \
ROBOTEST_FILTER=WifiCallingSettingsForSubTest
Change-Id: Ib6988e514bbad2023ba6cfc2e1d8734b68cabc31