Change StreamGobbler to Runnable to avoid unused Future

Change StreamGobbler to Runnable to avoid unused Future

It can be considered a programming error to create a Future<T>
but do nothing with that object. There is an async computation
happening and without holding and checking the Future for done
or exception the caller has no idea if it has completed.

FS doesn't really care about these StreamGobblers finishing.
Instead use Runnable with execute(Runnable), which doesn't
return a Future.

Change-Id: I93b66d1f6c869e66be5c1169d8edafe781e601f6