Revert "Add StagingManager service."

This reverts commit 9eb3b8ff.

Reason for revert: We are deciding for now not to make StagingManager a fully-fledged binder service, as it will only be accessed by PackageInstaller. We might re-evaluate this decision later if needed.

Bug: 122072686
Change-Id: Ic2a53fc92ddd7d7eeccc6a4a0117f28724346ec7