libbinder_random_parcel: splitout random binder

In preparation for more complicated random binders. Random
binders here are now symmetrical wrt FD logic.

This slightly changes the probabilities different kinds of
binders will used from random inputs, but it's not expected
to change coverage for arbitrary fuzzers.

Bug: 241923341
Test: binder_parcel_fuzzer
Change-Id: I9c3728cd2cde9a8abc62efecf1f567f366a38514