Fix flaky instrumentation tests, Compose timeouts, and NPE crashes

[temcguir]

This pull request introduces several stability fixes for Jetpack Camera App's instrumentation tests, which previously suffered from intermittent timeouts and crashes when running on emulators.

Changes:

• Fixed a race condition in PreviewViewModel: Events from the zero-capacity _captureEvents Rendezvous Channel were previously being blocked if consumed rapidly. We've split the event pipeline into _incomingCaptureEvents (an UNLIMITED capacity channel) and _outgoingCaptureEvents to cleanly bridge the events from the CaptureController to the Compose UI. This resolves ComposeTimeoutException flakes in Snackbar assertions and test runners.
• Accommodated emulator slowness: Increased DEFAULT_TIMEOUT_MILLIS in UiTestUtil.kt from 5 seconds to 15 seconds. This provides enough buffer for slower emulators transitioning back to the capture screen in CaptureModeSettingsTest without artificially flaking.
• Fixed a reproducible startup crash: Replaced instrumentation.context.applicationContext with instrumentation.targetContext.applicationContext inside CameraXCameraSystemTest. This resolves a 100% reproducible NullPointerException during test initialization.
• Code cleanups: Fixed Kotlin linter warnings, removed unused imports, and eliminated dead testing code in PreviewViewModel.

[gemini-code-assist]

Code Review

This pull request refactors several UI and camera system tests, adjusting timeouts, replacing forEach loops with standard for loops, and splitting the capture events channel in PreviewViewModel into incoming and outgoing channels. Feedback focuses on buffering the outgoing capture events channel to prevent potential deadlocks, simplifying semantics node checks in ComposeTestRuleExt.kt, removing redundant and unidiomatic 'val unused' assignments, and using Kotlin's native @Suppress annotation instead of Java's @SuppressWarnings.

[davidjiagoogle]

/gemini review

[gemini-code-assist]

Warning

You have reached your daily quota limit. Please wait up to 24 hours and I will start processing your requests again!