Enable a new rollback only when all child sessions succeeded (1/n)

Don't enable a new rollback until all its child sessions are
notified with success by SessionCallback#onFinished.

This change allows us to delete/remove a new rollback if any of its
child sessions failed before enabling it.

Bug: 134652027
Test: atest RollbackTest

Change-Id: Ie7cee8b0998a1df381a83219988a3876629fbcbb