Fix infinite recomposition in NavigationStackPlayground

[dfabulich]

I caught this while working on skiptools/skip-ui#292

Mutating state during init causes infinite recomposition.

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[marcprux]

Seems nasty! What tipped you off? In the past with infinite recomposition bugs, they've always had telltale continuous adb log output, but I don't see anything like that with the Path binding sheet in the Showcase app.

[dfabulich]

It was the Compose Testing tests in #83 that found it. In Compose Testing API tests, you call composeTestRule.waitForIdle() a lot. Sure enough, this test never went idle, because of its infinite recomposition. (I added logging to the playground to confirm it, and then logcat went crazy.) Fixing this bug fixed the test.