This moves the cache file deletion to a dedicated IntentService that runs
at the lowest possible priority. The cache cleanup does not need to happen
with any kind of priority, so it shouldn't delay the app start or take any
resources away from foreground processes.
This also changes the logic around the "Cache packages" preference. The
downloader always saves APKs, then if "Cache packages" is disabled, those
APKs are deleted when they are older than an hour.
This also simplifies Utils.deleteFiles() since the endswith arg is no
longer needed.
83ee0c8f0b