Persist staged sessions upon commit.

This change prevents staged sessions from being completely forgotten
upon commit, and leaves them in the XML storage even after commit. This
would allow resuming the state of the sessions upon reboot.

Bug: 118865310
Test: submitted a staged session, verified it was written on file on
reboot.

Change-Id: Iea600b66eac6f8655d671ae98a3a74f5003e5378