Make rollbacks available only after session commit.

In preparation for supporting rollback of multi-package installs, where
we need to use the SessionInfo to determine what the multi-package
install sets are.

Expire rollbacks relative to when they were committed, not when they
were first enabled.

Test: atest RollbackTest
Bug: 112431924

Change-Id: Ia8795813157ede3c1c5c5429d7b711db88c3e91a