Move the jni codes in a subdirectory of android_icu4j

The JNI codes are still part of core-icu4j jar, and thus kept
under android_icu4j/, until we rename/split the core-icu4j jar
to reduce confusion.

Bug: 142690826
Test: m droid
Change-Id: I4b06a0584fa95d9e6ff0135f7c3cd59694525273
24 files changed