EGL BlobCache: Add fuzzer for multifile

This fuzzer breaks up the incoming data buffer into
a key and value, then invokes the cache in a few
different ways.

Also includes one small fix to delete a buffer,
discovered while writing the test.

Test: /data/fuzz/arm64/MultifileBlobCache_fuzzer/MultifileBlobCache_fuzzer
Bug: b/261868299
Change-Id: I724584ddb5a92b4b33e371f22f4511cdeb965775