Do not enable rollback for apk session if no rollback found for staged session.

This fixes an issue where rollback may be made available for an apk within
a staged session even if there is no matching rollback for the staged session.

Bug: 139414218
Test: manual test: mixed apk/apex staged install created; rollback expired for the apex;
device rebooted; checked that the new code was activated and rollback not enabled for
the apk part.
Test: atest StagedRollbackTest
Test: atest CtsRollbackManagerHostTestCases

Change-Id: Iea72b7dc74f79c81627ee7eee0ef91ce6676ae52