Backport of ag/14170751 and ag/14170752.

1. An available rollback shouldn't be deleted when its session expires.
2. Add a test to check we don't crash or delete a rollback when its
   session expires.

Bug: 181087240
Bug: 185132440
Test: atest StagedRollbackTest

Merged-In: I3c2b718905d2d7619d6f299ee5402fd858de030e
Merged-In: I1bd786b38e2df1f4bdc80d0d371d5d2d52e21e06
Change-Id: Id55f1bde0ee69a6f26bc8e109918fb26d1da4e3f