binder_parcel_fuzzer: support write functions

This generalizes the append parcel fuzzing functionality,
for additional hardening.

Fixes: 328161314
Test: manual, run binder_parcel_fuzzer for 3min
Ignore-AOSP-First: internal only for security hardening

Change-Id: I1f470e0d56241327bff7422d061912b592cc0ddc