Fix intermittent build failures with droiddoc generated sources

Jack builds fail very rarely with missing droiddoc generated
source files.  This happens because jack builds have an extra
rule that does a quick check of the sources and touches a
jack.check.timestamp file.  The manual dependencies for the
generated sources didn't cover this case.  If the sources are
partially generated when the jack.check.timestamp rule runs,
the rule can see partial sources and fail.  The same problem
might also happen for javac builds after
https://android-review.googlesource.com/c/platform/build/+/404743
separated the source list generation rule out of the
$(full_classes_jar) rule.

Use LOCAL_ADDITIONAL_DEPENDENCIES instead, which covers these
cases and will be updated to cover future changes.

Bug: 65255877
Test: m -j ANDROID_COMPILE_WITH_JACK=true out/target/common/obj/JAVA_LIBRARIES/android.test.runner.stubs_intermediates/jack.check.timestamp
      make sure sources are generated

Change-Id: I4466b3128eb0bba6447c24abf921a6655a3d451f