binder_parcel_fuzzer: split out random_parcel.h

This creates libbinder_random_parcel which can create a random libbinder
parcel, complete with random fds/random binder objects, for use in other
fuzzers.

Future considerations:
- also export NdkBinderParcelAdapter, for use fuzzing libbinder_ndk
  users
- implement similar functionality for libhwbinder

Bug: N/A
Test: binder_parcel_fuzzer
Change-Id: I4943c5e8b6662a8155dc42109eda245f35eedef8