Preserve active staged sessions.

If a staged session is pending, we should never garbage collect it.
Leave it to the installer to abandon the session if needed.

This is a partial revert of Ifaccf50e744a97e30a4fda0e47b88d7dc71adcf8.
In reality, this logic was correctly preserving non-final staged
sessions, and it was the condition in sealAndValidateIfNecessary that
previously caused the erroneous early garbage collection of staged
sessions in final state.

Fix: 132444450
Bug: 132444450
Test: staged a session, manually modified update timestamp, verified
that it's correctly applied on reboot. manually modified update
timestamp again, verified that it's correctly garbage collected.

Change-Id: I5fc568357fb8ec483d52056f5a805b09149cfec9