[pm] cleanup unused PackageManagerInternal APIs for loading progress

Launcher gets loading progress from LauncherActivityInfo directly and
does not need to use the listener. Removing unused methods, plus a bit
of code cleanup.

BUG: 175135603
Test: atest CtsInstalledLoadingProgressHostTests
Test: atest FrameworksServicesTests:com.android.server.pm.IncrementalStatesTest
Change-Id: Icf96f5ca23e567396cfc233725619ba91a953c97