Merge changes from topics "abort-individual-session", "apk-checkpoint",...

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