Fix screenshot tests: freeze animation clock to prevent AppNotIdleException

Disable mainClock.autoAdvance in captureForDevice and captureMultiTheme
before calling onRoot(), so infinite animations (e.g. NiaLoadingWheel)
don't block Espresso idle synchronization in Compose 1.8.0.

core/screenshot-testing/src/androidMain/kotlin/com/google/samples/apps/nowinandroid/core/testing/util/ScreenshotHelper.kt

@@ -110,6 +110,9 @@ fun <A : ComponentActivity> AndroidComposeTestRule<ActivityScenarioRule<A>, A>.c
         }
     }
 
+    // Freeze animations so infinite transitions don't block Espresso idle sync
+    this.mainClock.autoAdvance = false
+
     // Run Accessibility checks first so logging is included
     val accessibilityException = try {
         this.onRoot().checkRoboAccessibility(
@@ -201,6 +204,9 @@ fun <A : ComponentActivity> AndroidComposeTestRule<ActivityScenarioRule<A>, A>.c
         }
     }
 
+    // Freeze animations so infinite transitions don't block Espresso idle sync
+    this.mainClock.autoAdvance = false
+
     // Create permutations
     darkModeValues.forEach { isDarkMode ->
         darkMode = isDarkMode