Add ICU4J APIs for Android

Add the planned set of new I18N public APIs for
Android N.

This APIs consists of a whitelisted subset of ICU4J APIs
repackaged under android.icu (not com.ibm.icu).

It includes no deprecated, provisional, internal methods from
ICU 56.

There may be future work to improve the ICU4J JavaDoc text
for Android, but the API surface area should be mostly
stable at this point.

Bug: 22023363
Change-Id: Ib4eca03c71c594ab098edcae3347f2ecc2c35c72