Don't enable rollback again in pre-reboot verification

See b/167645815#comment1 for details.

1. In notifyStagedSession(), rollback is enabled for the 1st time.
2. In verifyStagedSession(), rollback is enabled again and
   FileAlreadyExistsException is thrown.

Remove the INSTALL_ENABLE_ROLLBACK flag if the verification is kicked
off by a staged install.

Bug: 167645815
Test: atest RollbackManagerHostTest#testApkOnlyMultipleStagedPartialRollback
      and check logcat messages that no FileAlreadyExistsException is thrown
Change-Id: Id8a96a1dd790331170e2a7e201634b23d20424c6