Fix FsUtils.readDataFromUrl() to do the network request on a background thread

This is required now that doing network requests on the main thread
throws NetworkOnMainThreadException.

Also updates TestsListPreloaderThread to create the FileFilter lazily
and removes a superfluous FileFilter param from the Summarizer
constructor.

Change-Id: If15fddac934cd8be611f6693b25837657abbddc3