Add a state for Staged Sessions.

The state is only kept up-to-date for staged sessions. Clients are in
charge of checking whether the session has the isStaged bit set, before
considering reading the StagedSessionState associated to that session.

Test: retrieved mock staged session from a small app and verified state
is correctly set.
Bug: 118865310
Bug: 112669193
Bug: 120487127
Change-Id: I03590476dc353fee6d6edffb7ae579a9f9664664