ICU-22444 Remove "unknown" from Calendar.getKeywordValuesForLocale

Remove CalType enum value UNKNOWN and use null for unknown CalType
This value is an internal enum and the only place use it is inside Calendar.java
Use null for that instead (same as C++)

icu4j/main/classes/core/src/com/ibm/icu/impl/CalType.java

@@ -26,9 +26,7 @@ public enum CalType {
     ISLAMIC_UMALQURA("islamic-umalqura"),
     JAPANESE("japanese"),
     PERSIAN("persian"),
-    ROC("roc"),
-
-    UNKNOWN("unknown");
+    ROC("roc");
 
     String id;
 

icu4j/main/classes/core/src/com/ibm/icu/util/Calendar.java

@@ -1814,14 +1814,14 @@ private static CalType getCalendarTypeForLocale(ULocale l) {
                 }
             }
         }
-        return CalType.UNKNOWN;
+        return null;
     }
 
     private static Calendar createInstance(ULocale locale) {
         Calendar cal = null;
         TimeZone zone = TimeZone.forULocaleOrDefault(locale);
         CalType calType = getCalendarTypeForLocale(locale);
-        if (calType == CalType.UNKNOWN) {
+        if (calType == null) {
             // fallback to Gregorian
             calType = CalType.GREGORIAN;
         }

icu4j/main/tests/core/src/com/ibm/icu/dev/test/calendar/CalendarRegressionTest.java

@@ -2186,6 +2186,9 @@ public void TestGetKeywordValuesForLocale(){
         String[] ALL = Calendar.getKeywordValuesForLocale("calendar", ULocale.getDefault(), false);
         HashSet ALLSET = new HashSet();
         for (int i = 0; i < ALL.length; i++) {
+            if (ALL[i] == "unknown") {
+                errln("Calendar.getKeywordValuesForLocale should not return \"unknown\"");
+            }
             ALLSET.add(ALL[i]);
         }