More robust multi-threaded downloading.
ApkDownloaders keep track of a unique id. This id is passed through with
each event coming from the downloader. When progress events are received,
if they don't have the same id as the current downloader, they are ignored.
When returning to the app details screen after leaving, if a download
completed in the mean time, automatically show the install screen. Otherwise,
when you, e.g. return to your devices home screen during a download, the
download keeps occuring in the background, but we are unable to make use of
the resulting downloaded file.