Unbundle i18n API from android-non-updatable

This removes the i18n source from the non-updatable stubs which removes
them from the frameworks/base/core/api files too. The API is still
protected by both the build and Gerrit review as the API is specified
in the external/icu/android_icu4j/api/public/ directory.

The i18n stubs source is used instead of the implementation source for
generating the JavaDoc. That revealed a minor issue with metalava which
is fixed in this topic.

The i18n stubs library was statically included alongside the conscrypt
stubs library so it will still appear in the monolothic android.jar.

Bug: 174344410
Test: m offline-sdk-docs
      - check that i18n classes are present
      m android_stubs_current android_system_stubs_current android_test_stubs_current
      - check that i18n classes are present in each jar
      m droid
Change-Id: I744c8bac54a8a5518ca0eca2a7700abbfa0997f4