Rewrite handling of failed sessions (2/n)

We want to delete rollbacks for failed sessions as long as parent or
child session id matches. For the sake of defensive programming, we
check #isEnabling to ensure we won't delete rollbacks that are already
made available or committed.

Bug: 149069841
Test: atest RollbackTest StagedRollbackTest
Change-Id: Ie95789c9c87f91d2cca84fede621bd17a6a9da18