Allow attaching multiple unarchivalListeners to a single session.

This case can happen if multiple apps request unarchival for the same
app. E.g. a user might first tap the icon on the launcher screen, and
later on another app calls the requestUnarchive() API manually. This will
be a rare case though.

Also remove some outdated TODOs that were already addressed.

Test: PackageInstallerArchiveTest
Bug: 313601388
Change-Id: If8a7bc2886b84b272be27260172fee944f556795