Fuzz corpus entries should not go in a subdirectory.

Corpus files called 'FOO' incorrectly go into a subdirectory in the
exported zipfile, e.g.:
my_fuzzer/corpus/FOO/FOO

This patch removes that extra, unnecessary intermediate directory,
e.g.:
my_fuzzer/corpus/FOO

Bug: 142432347
Test: m fuzz && unzip -l out/soong/fuzz-host-x86_64.zip
Change-Id: Id23d4dd2d8322a8ad0234f8acb6878ea4dc3c6e4