libfmq: add mode where message size is only known at runtime am: 749e1da036 Original change: https://android-review.googlesource.com/c/platform/system/libfmq/+/3026403 Change-Id: I1f08598f1b09eefc0dec17da74af0614fd55371c Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>