Fix handling of multi-package enable rollback failures.

Failure to enable rollback for one of the packages in a multi-package
install should fail to enable rollback for all packages in the
multi-package install. That was not the case prior to this CL, which
could lead to partial rollbacks.

Keep track of how many child sessions we expect to enable rollback for
and double check that we succeeded before making the rollback available.
This involves some cleanup of how we track parent vs child sessions when
enabling rollback.

Bug: 128656191
Test: atest RollbackTest, with new test added.

Change-Id: I737896cdc1915396748c5c1959b5397af793258a