Treat kotlinx.collections.immutable.ImmutableCollection as stable

[mcpiroman]

This will make parameters that have more general type of ImmutableCollection rather than List or Set stable.

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[jimgoog]

👍 Seems reasonable to me. CLA required, see instructions from CLA bot. Also, lint is complaining that there is no unit test stanza, you can amend your commit message to say Test: unit tests that will make lint happy.

[mcpiroman]

Done.