Merge changes from topics "abort-individual-session", "apk-checkpoint", "rebrand-to-revert", "supportsCheckPoint"
* changes:
Enable filesystem checkpoint for staged apk installation
Allow staging multiple sessions with non-overlapping packages (apk+apex)
Replace abortActiveSession in StagingManager with revertActiveSessions
Allow StagingManager to abort individual staged session
Prevent staging multiple sessions on devices not supporting checkpoint
Rebrand the term "rollback" to "revert" for apexd