[DX] Moved Security constants to a final class instead of a long named interface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#4188

This PR is based on feedback from the documentation repository. The DX suggestion was to rename the new `SecuritySessionStorageInterface` to `Security`. This would make it easier to use the constants before 2.6 is released. In this PR I have also update all usages of this constant because an open PR is now merged which used those constants.

List of changes:
 - SecurityBundle, usage of constants
 - Security Component (core & http), usage of constants
 - Tests, usage of constants
 - Added a test to verify the sync from `Security` to `SecurityContextInterface` for BC purposes