limit extra fds in random parcel

Fuzzers are hitting open fd limit for large inputs.
Restricting number of fds which are added in extraFds.

Bug: 257828440
Bug: 260119717
Bug: 258307992
Bug: 260738157
Test: m libbinder_random_parcel
Change-Id: I401ebe1dd754f891cbddd0d982ab4439984c3e3f