New DownloaderService

This merge request is too big, but I have all this work complete, so I'm submitting it now for review. If there are bigger issues with parts, I can rebase it down to the uncontroversial bits to get things merged.

This replaces ApkDownloader, AsyncDownload, and AsyncDownloadWrapper, and puts the whole APK download procedure into a custom Service that is based on the source code for IntentService. It can't be a subclass of IntentService because it needs to be cancelable. This does not yet add back a notification for the downloading, e.g. #592 (closed). That was handled before by the Android DownloadManager stuff, and was not replaced since that was disabled.

My current implementation does not filter out duplicate requests like discussed in #601 (closed). While its possible to do, I think it'll complicate the code a lot, and I really think that should be handled elsewhere. The UI should prevent the possibility of the user being able to submit duplicate install requests. If not, even if DownloaderService filtered them out, it would still be a buggy UX since the user would be clicking install again or something like that even though the install was in progress.

This also moves the APK verification logic to the Installer side. The downloading side can check the file size to see if the whole thing is downloaded. And to be extra safe, it should not be possible to submit an APK for installation without it going through the verification procedure. So the only method for installing APKs, Installer.install(), is where the verification now happens. Also, the installer now always copies the APK to be installed into the safe location RE: the Cure53 audit issue. This way, the APK download and cache dirs can be merged into one, making resumable downloads and cache management easy.

ping @mvdan @pserwylo @dschuermann more comments in the commit messages