binder_parcel_fuzzer: avoid timeouts

libFuzzer was causing this test to timeout by feeding it, say, 600kb of
input, corresponding to 150,000 instructions to run. This kind of input
isn't interesting and is distracting from real results, so artificially
cutting the maximum input size to 50kb (~ 17,500 instructions).

Fixes: 142617274
Test: binder_parcel_fuzzer
Change-Id: I320ec57a9d1827ecaec035a3b5414bc4f97c0620