InstallManagerService

This provides an over-arching Service for managing the whole install process, from checking the cache, downloading files, handling the notification. Ultimately, it should probably also handle starting and tracking progress of the final installation steps.

Note: this does undo some of the Notification handling stuff, putting it back to one notification per APK. I did that to get that part working OK for the short term, giving us time to figure out what the whole picture should look like. I think @pserwylo has it pretty well sketched out in #592 (closed). But I have no strong feelings about the notification stuff for 0.100, so I'm happy to shape this MR accordingly, provided its only a little work.