As c9ae7c8 showed, changing internal ids without being aware of the
possible impact might easily lead to break the external API.
While ids could be fixated by adding respective tests, an even more
elegant solution is to make the mapping from internal ids to external
ids explicit, similar how it is already done for other voice hint
formats.
To underline the purpose of the mapping even more, the
respective method is renamed appropriately.
Test Plan:
- `./gradlew test`
- Export a complex route in BRouter-Web and check voice hints have not
been changed.
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