libbinder_random_parcel: hide fill parcel data

We have no external users of this function, and I'd generally
recommend against using it (it doesn't test things in RPC mode
currently, and it limits the branches that would be explored).
Hiding it for now.

Bug: 224646709
Test: run binder_parcel_fuzzer
Change-Id: Iafb61b9e201861f2f87de2f99217294b0d532881