Test fix: Add MmsPreferenceController to whitelist

Add it to grandfather_slice_controller_not_in_xml since right now
it only exists in mobile_network_settings_v2.xml, not v1.

This should be fixed automatically once we remove all old code.

Bug: 130222866
Test: RunSettingsRoboTests
Change-Id: Ib0bc976fd31818d7b39ca9f3e7427859fd7175ff