Merge changes from topic "reiwa_o_mr1_cts" into oreo-mr1-cts-dev

* changes:
  DO NOT MERGE [CtsIcuTestCases] Old Android releases can optionally support the new Japanese era
  DO NOT MERGE Cherry-pick: ICU-20536 Japanese era Reiwa support in ICU4J 58
  DO NOT MERGE Cherry-pick: ICU-20536 Japanese era Reiwa support in ICU4C 58