Do not disable maintenance mode after running a backup.

[stevenvdp]

Description

Keep the initial maintenance mode state before running the backup.
Only disable maintenance mode if it was not setted before.

Github issue: #7951

Fixed Issues (if relevant)

1. Running setup:backup with maintenance mode already on, disables after completion. #7951: Running setup:backup with maintenance mode already on, disables after completion.

Manual testing scenarios

1. run > bin\magento maintenance:enable
2. run > bin\magento setup:backup --db
3. when backup is complete, you don't see:
   "[SUCCESS]: DB backup completed successfully.
   Disabling maintenance mode"

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[miguelbalparda]

It seems you and @jokeputs are working on the same and #11052 incudes several other fixes for this. Maybe you can work together on the same PR?