Add incoming threads to binder_bpBinderFuzz

If the RpcSession associated with a binder has no incoming threads, then
calling linkToDeath on it is fatal.

Test: binder_bpBinderFuzz
clusterfuzz-testcase-minimized-binder_bpBinderFuzz-6220735032197120
Bug: 239917056

Change-Id: I471b72e55c240ac8344ebc8506c32e60c61d13bf
1 file changed