Avoid hidden DataFormat.is24Hour field access

DateFormat.is24Hour is not part of the CorePlatformApi but
DateFormat.set24HourTimePref(Boolean) is. Switch over.

Bug: 113148576
Test: build
Change-Id: I27103890f94cfe3679f386ea33b0450b453a7625

core/tests/coretests/src/android/text/format/DateUtilsTest.java

@@ -117,7 +117,7 @@ public class DateUtilsTest {
     @Test
     public void testFormatSameDayTime() {
         // This test assumes a default DateFormat.is24Hour setting.
-        DateFormat.is24Hour = null;
+        DateFormat.set24HourTimePref(null);
         Date date = new Date(109, 0, 19, 3, 30, 15);
         long fixedTime = date.getTime();