Show time zone standard name in FixedOffsetPreferenceController

- Among all fixed offset time zone, only Etc/UTC has a time zone
standard name. The name should be shown in both picker and preference's
secondary text (summary).

Bug: 74338688
Test: m RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.datetime.timezone
Change-Id: I10c140f6054ab9acf6e0fb44528adef8b28fa23c