Generate specific message for build flag declarations artifact

Many of the fields in `flag_artifact` are not valid in
the all_build_flag_definitions artifact.

Bug: 328495189
Test: manual
Ignore-AOSP-First: cherry-pick
Merged-In: I00eab1ef76f67f7db2118a6fc0d5771e3dd39fbb
Change-Id: I00eab1ef76f67f7db2118a6fc0d5771e3dd39fbb