Limit visibility of libfmq_rust while under development am: 4bbbba517a am: 2e20bece80

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

Change-Id: I3868052f770bd1c11ac854ccbeba52d2963c29e9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>