Re-plumb fuzz corpora.

Re-plumb the fuzz corpora through an intermediates directory.
Previously, due to the way that LOCAL_TEST_DATA is implemented, we would
break when corpora were included:
 a) outside of a corpus/ directory, or
 b) not directly under the module directory (i.e. my_module/subdir/corpus)

Corpora are now written into an intermediates directory so that they
play nice with LOCAL_TEST_DATA, because unfortunately the key-value pair
required there is not as simple as source:destination.

Test: m example_fuzzer && ls \
$ANDROID_PRODUCT_OUT/data/fuzz/arm64/example_fuzzer/corpus
Bug: N/A

Change-Id: I5888b06ed349781c6712f1ae77cc3582283a9552