Add repackaged version of the ICU tests

Adds a new android-icu4j-tests target to build the repackaged
tests. The repackages tests were generated by running the
following script which was modified to repackage the tests in
addition to the source and sample.
    tools/srcgen/generate_android_icu4j.sh

Bug: 22023363
Change-Id: I3c6e65ba6a4d24055343460a023d6671585146f5
diff --git a/README.android b/README.android
index 5f8ff53..e03e21a 100644
--- a/README.android
+++ b/README.android
@@ -3,7 +3,12 @@
 Directories
 ===========
 
-icu4j/,icu4c/   - ICU4J and ICU4C with Google and Android-specific patches applied.
+icu4c/          - ICU4C with Google and Android-specific patches applied.
+
+icu4j/          - ICU4J with Google and Android-specific patches applied.
+                  After patching this make sure to run the following to update
+                  the repackaged version used in Android and CTS tests.
+                      tools/srcgen/generate_android_icu4j.sh
 
 android_icu4j/  - A transformed copy of icu4j source repackaged into android.icu.
                   This is the copy of ICU4J used in the Android platform.