Declare a public field for new Japanese Era Reiwa

Integrate OpenJDK commit for new Japanese Era name:

 * 8174268: Declare a public field in JapaneseEra for the era starting May 2019
   http://hg.openjdk.java.net/jdk/jdk/rev/6a4abdb6749c

Bug: 149437171
Test: CtsLibcoreTestCases CtsLibcoreOjTestCases
Change-Id: I1cbc9a12744d3cea214ee3b4a2872a9ad144e147