Made DigitList public for testing

* Added new rule to make classes visible for testing.
* Updated tools/srcgen/generate_android_icu4j.sh to delete
  previous work around for DigitListTest.
* Added DigitListTest to android_icu4j

Bug: 33448125
Test: tools/srcgen/generate_android_icu4j.sh
Change-Id: I5126a51b895e877cdac5adad66cfce60115016e4
5 files changed