Migrate core.current.stubs generation to Soong.

Since Soong now supports droiddoc generation. Move this to Soong.

Test: Generated old jar; Generated new jar;
find /tmp/new -type f -name "*.class" -exec md5sum {} \+ | ( cd /tmp/old; md5sum -c --quiet )

Bug: b/70351683

Change-Id: I9cef9ca40dea32c7cb6087dae18340f4b23142fa