Test: Add a fuzzer for libmedia's metadata component

libmedia_metadata_fuzzer attempts to add random key/value pairs of the eligible types using metadata's appendBool and appendInt32 methods to a Metadata object. The fuzzer also calls the appendHeader() method to add a header, the updateLength() method during each iteration of appending key/val pairs and a resetParcel() method at the end.

Change-Id: I569c39954702f848bf5496528f7c3eb888c33ccc