binder_parcel_fuzzer: construct parcels w/ objects

Better coverage, since before we didn't test read methods that might get
ahold of valid objects. This is in preparation of exporting and using
these randomly generated parcel objects elsewhere.

Bug: N/A
Test: binder_parcel_fuzzer
Change-Id: Iac53cbe0437cea39a0d94177abe4e33d50eb189e