libbinder: organize fuzzers

Several class-specific fuzzers were added independently from our parcel
fuzzer. Granted, the parcel fuzzer is technically for the Parcel class,
it also fuzzes libbinder_ndk and (for convenience) libhwbinder, so I'm
leaving it as separate.

Bug: 182938024
Test: run fuzzers
Change-Id: Ic2e461c12439c01e3e3ffd7265a94593132c6ca6