Allow alternative time zone ID to be set device's zone.

If America/Godthab is set as device time zone, display it as if
America/Nuuk is chosen.
If time zone is known, but is not an alternative and not shown
in time zone picker, region will de derived from user's locale.

Bug: 155738410
Test: atest com.android.settings.datetime.timezone.model
Test: set persist.sys.timezone via adb and checked
Date & Time screen

Change-Id: I168fb4319e144dbe9e2496d06a681d4c09b411c0