Fix a flaky test (5/n)

Sometimes the rollback doesn't become available right
after install is completed. We need to retry and wait.

Bug: 195517333
Test: atest StagedRollbackTest#testRollbackRebootlessApex \
      --rerun-until-failure 99
Change-Id: I7944d67529328839e9a59337ed7e60abd43d35d4