Test for forcing Gregorian as default Calendar

Gregorian is the default calendar for all locales in Android.
This CL adds the test to verify it.

2 data sources of 'default' calendar
- calendarPreferenceData property in supplementalData.txt.
  It's used by classes, e.g. Calendar.
- calendar/default property in icu4c/source/data/locales.
  It's used by classes, e.g. RelativeDateTimeFormatter
  However, there is no direct way to obtain the calendar type
  via the RelativeDateTimeFormatter API. The test obtains and
  verify the calendar type via ICU data resources.

Bug: 80294184
Test: ICUCalendarTest
Test: CalendarTest
Change-Id: Id2f441973e1566cb4a82a29350e98ac872fc20fe