[pm/launcher] add back progress listening in PackageManagerInternal

(This partially reverts ag/13481636)

Turns out Launcher still uses
LauncherApps.onPackageLoadingProgressChanged(), which relies on the
internal listener exposed via PackageManagerInternal. Adding it back.
We don't need the unregister method, because all the progress listeners
will be automatically removed when the app is fully loaded.

BUG: 175135603
Test: manual with incremental install via Phonesky
Change-Id: Ief09d3c7775080f35568b6ec51d6351a9373f04e