Expose more ICU4J APIs.

Expose more classes/methods, mostly those added in ICU4J 57.

Additionally EthiopicCalendar is now un-hidden which was hidden
accidentally and should have been in N.

Bug: 30825380
Test: m checkapi
Change-Id: Ib3bea26a9b246401d33cc0ccc0f91768d1fe1286