Limit visibility of libfmq_rust while under development am: 4bbbba517a

Original change: https://android-review.googlesource.com/c/platform/system/libfmq/+/3118152

Change-Id: Id327ddb119b22175599685bc2bf08ed1d6e4d336
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>