Use isShownInPickerAt instead of isShownInPicker.

The latter is being removed.
The date(1 Jan 2019 00:00) was a cut-off date used in
SettingsUI. This change moves it to reuse that date without
duplication.
Affected method is used in setup wizard to answer "What time zone
IDs should we show for a country?". The question is vague, but
the change keeps user experience consistent across device w/o
breaking method contract.

Bug: 155738410
Test: presubmit
Change-Id: Ib139b316e5909cab18d015b7c163bb9c79bcef5b