overhaul DefaultInstaller to use PackageInstaller API

There seems to be quite a few changes to PackageInstaller on Android Q, see the documentation for things like installExistingPackage, getStagedSessions, and com.android.packageinstaller.notification:

https://developer.android.com/reference/android/content/pm/PackageInstaller

@commonsguy can we ask you for some advice? Anything scary and/or urgent we need to address? I didn't see anything yet, but I still need to dive into your commonsware materials on this topic.

2021-05-20 update: ACTION_INSTALL_PACKAGE will soon stop working, and PackageInstaller now has a nice setRequireUserAction feature to allow background updates. So the default Installer should switch to PackageInstaller APIs and use that.