Fix docs build after switch to CorePlatformApi

Fix docs build after switching the build to use CorePlatformApi
stubs. Apparently previously it was fine to link to classes not
in the public API / CorePlatformApi, but it's not any more.

Bug: 117831958
Test: m docs
Test: generate_android_icu4j.sh
Change-Id: I5944e4f1cd277b8f8e96d35d56e63ed93fdc54ea
2 files changed