binder_parcel_fuzzer: avoid consuming all provider

In doReadWriteFuzz, it should not call fillRandomParcel,
as this consumes the entire provider. It's meant to
always start with an empty Parcel, but I forgot to
remove this.

Ignore-AOSP-First: fuzzer work
Bug: 328161314
Test: run fuzzer, starts finding crashes
Change-Id: I1ce474a4e39464fd53f6cd9c440b40bd128fada1