Fixes #19170: [java] Avoid name collision for models named "Schema" when annotation library it's not used

[gtsystem]

This change avoid the collision of models named "Schema" in the case AnnotationLibrary swagger2 is not set.

In short, the Schema key is now added to importMapping only when necessary.

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cc
@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)

[martin-mfg]

Hi @gtsystem, thanks for the PR!

1. Some months ago, I had opened [JAVA][SPRING] add imports & primitives to reserved words #16713, which also solves the problem that is being addressed here, but in a more generalized way. If we could merge #16713, this PR here would be not necessary anymore.
2. Otherwise, I think the suggested fix you (@gtsystem) described in [BUG][Java] Schema name collision with template imports #19170 - adding "Schema" to reserved words - would be the preferred solution. It would work regardless of swagger2 annotations being used or not.
3. The third choice would be merging this PR. I confirm it does fix the problem.

I would prefer solution 1 listed above. If that's not possible, I'd still prefer option 2 over option 3.

@wing328, could you kindly merge #16713?