Replace ViewHelpers with explicit measure/layout; childCountTest uses view.post polling

[EmilioBejasa]

Summary

• Remove ViewHelpers from IsolatedTest.kt (constructViewTest, childCountTest) and ScreenshotTest.kt (testSimpleTextView); replace with explicit view.measure() / view.layout() calls
• Replace 30s poll loop in childCountTest with a self-rescheduling Runnable posted via view.post(). WindowAttachment.dispatchAttach injects a real Handler (bound to the main Looper) into AttachInfo, so view.post() routes to the main MessageQueue. The Runnable checks childCount every 50ms on the main thread and completes a CompletableFuture<Int> when children appear; the test thread blocks on future.get(5s).
• Remove childCountScreenshotTest — it was a duplicate of childCountTest plus a screenshot; the screenshot half will live in a follow-up PR
• Capture startTask return value and assertGoodTask it before waiting on the future, so the bg executor is fully drained before polling begins

Why view.post works here

WindowAttachment.dispatchAttach (screenshot-tests-for-android) doesn't add the view to a real window — it uses reflection to inject a fake AttachInfo. That AttachInfo is constructed with new Handler() on the main thread, so view.post(r) calls attachInfo.mHandler.post(r), which posts to the main Looper. OnGlobalLayoutListener does not work here because there's no real ViewRootImpl driving layout traversals.

Test plan

• simpleTest passes
• constructViewTest passes
• childCountTest passes (no sleep, no CountDownLatch, one setup runOnMainSync)
• recordDebugAndroidTestScreenshotTest succeeds; addViewHookTest screenshot shows "Hello" (not blank)
• verifyDebugAndroidTestScreenshotTest passes

🤖 Generated with Claude Code