Improved testing around DateFormat.is24Hour

Adds a test that proves the SHORT/MEDIUM time formats are not
cached and the DateFormat.is24Hour setting is correctly honored.

Bug: 31762542
Test: run cts -m CtsLibcoreTestCases -t libcore.java.text.DateFormatTest
(cherry picked from commit 0c0bab19d12ac8310ade76060e2fe42814c387d4)

Change-Id: I4a1db0b9bb6e13f553b7a272471c0e09b156a905
1 file changed