MojoLPM: Fixing and refactoring MediaStreamDispatcherHost fuzzer.

The MediaStreamDispatcherHost fuzzer wasn't correctly binding the
interface to be fuzzed. This fixes that and refactors the fuzzer to
remove shared code.

Change-Id: I219a4654c9f6b77ddb5f1f6fd2a6c6b9cf0ebe18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3697084


Reviewed-by: Chris Bookholt <bookholt@chromium.org>
Commit-Queue: Mark Brand <markbrand@google.com>
Cr-Commit-Position: refs/heads/main@{#1012854}