libbinder_parcel_random: internalize RPC parcels

Recently, I changed binder_rpc_fuzzer to optional construct an RPC
parcel. However, it doesn't make sense for individual users of
libbinder_parcel_random to concern themselves with things like this.

Bug: 182938024
Test: N/A
Change-Id: I0d1cc5bafd0031f8cddf57a82a13f39cd36369e1