Perform user data back up in post install stage

Currently the backup of user data is done in the enable rollback stage,
during which there is no guarantee that the package being backed up is
not currently running. Moving the backup to the post install stage will
guarantee that the package is not running.

Test: atest RollbackTest
Test: atest StagedRollbackTest
Bug: 124032231
Change-Id: I4b42a0c5ade1645585d1d6f698637df950d05c72